NASHVILLE, Oct. 1 — At the Riverbend Maximum Security Institution here, through a set of double doors next to several vending machines, a gurney stands ready to deliver prisoners to their executions by lethal injection.

Just about every aspect of the death penalty provokes acrimonious debate, but this method of killing, by common consensus, is as humane as medicine can make it. People who have witnessed injection executions say the deaths appeared hauntingly serene, more evocative of the operating room than of the gallows.

But a growing number of legal and medical experts are warning that the apparent tranquillity of a lethal injection may be deceptive. They say the standard method of executing people in most states could lead to paralysis that masks intense distress, leaving a wide-awake inmate unable to speak or cry out as he slowly suffocates.

In 2001, it became a crime for veterinarians in Tennessee to use one of the chemicals in that standard method to euthanize pets.

The chemical, pancuronium bromide, has been among those specified for use in lethal injections since Oklahoma first adopted that method of execution in 1977. Only now, though, is widespread attention starting to focus on it.

Spurred by a lawsuit by a death row inmate here, advances in human and veterinary medicine, and a study last year that revealed for the first time the chemicals that many other states use to carry out executions, experts have started to question this part of the standard lethal injection method.

Pancuronium bromide paralyzes the skeletal muscles but does not affect the brain or nerves. A person injected with it remains conscious but cannot move or speak.

In Tennessee and about 30 other states, the chemical is used in combination with two others. The other chemicals can either ease or exacerbate the suffering the pancuronium bromide causes, depending on the dosages and the expertise of the prison personnel who administer them.

A judge here recently found that pancuronium bromide, marketed under the trade name Pavulon, has "no legitimate purpose."

"The subject gives all the appearances of a serene expiration when actually the subject is feeling and perceiving the excruciatingly painful ordeal of death by lethal injection," the judge, Ellen Hobbs Lyle, wrote, describing the worst-case scenario. "The Pavulon gives a false impression of serenity to viewers, making punishment by death more palatable and acceptable to society."

A simpler and more humane alternative to the three-chemical combination, many experts agree, is the method usually used in animal euthanasia: a single lethal dose of a barbiturate called sodium pentobarbital.

Dr. Sherwin B. Nuland, who teaches medicine at Yale and wrote "How We Die" (Knopf, 1994) said he was baffled to hear that pancuronium bromide was used in executions.

"It strikes me that it makes no sense to use a muscle relaxant in executing people," he said. "Complete muscle paralysis does not mean loss of pain sensation."

Dr. Nuland, who described himself as a cautious supporter of the death penalty, said a humane death could be achieved in other ways, including by using the other two chemicals in the standard method, without the pancuronium bromide.

The challenge to the use of pancuronium bromide was brought in chancery court here by Abu-Ali Abdur'Rahman, who is on death row for a 1986 murder. Judge Lyle wrote that the use of the chemical "taps into every citizen's fear that the government manipulates the setting and gilds the lily." But despite her misgivings, she ruled that the use of the drug did not violate the Constitution's ban on cruel and unusual punishment, because it was widely used and because "there is less than a remote chance that the prisoner will be subjected to unnecessary physical pain or psychological suffering."

The case is on appeal.

Mr. Abdur'Rahman, 52, is being held at the Riverbend prison, along with 92 other death row inmates. He is short and slight, and his long beard has turned gray. He spoke to a visitor through thick glass.

"They're saying I'm less than an animal," Mr. Abdur'Rahman said. "The poison they put into our veins needs to be challenged. Had my attorneys not researched this, I doubt very much it would have come to light."

The American Veterinary Medical Association condemns pancuronium bromide when it is the sole chemical used or when it is used in combination with the usual animal euthanasia drug, sodium pentobarbital. That is because, an association report in 2000 said, "the animal may perceive pain and distress after it is immobilized."

Lethal injection is now the dominant way Americans are executed. It is used in all 38 states that have the death penalty except Nebraska, which uses electrocution. In 10 states, prisoners may choose between lethal injection and a second method, including hanging, firing squad, electrocution and lethal gas.

In most methods of lethal injection, pancuronium bromide is the second drug in a three-chemical sequence.

The first is sodium thiopental, a so-called ultra-short-acting barbiturate. It can be effective for only minutes. In surgery, it is used to induce rather than maintain anesthesia. Doctors like it because patients who encounter immediate complications awaken quickly enough to be saved.

The third is potassium chloride, which stops the heart and causes excruciating pain if the prisoner is conscious.

"It would basically deliver the maximum amount of pain the veins can deliver, which is a lot," Dr. Mark J. S. Heath, an anesthesiologist who teaches at Columbia, testified at a hearing for Mr. Abdur'Rahman.

One problem with the combination of chemicals, Dr. Heath said in an interview, is that the sodium thiopental could be inadequate or wear off. That would leave the prisoner conscious, paralyzed, suffocating and subject to extreme pain from the potassium chloride.

"You're in a chemical tomb," he said.

The possibility of improper doses or sequences and of bungled injections is increased, Dr. Heath said, because doctors may not take part in executions under most codes of medical ethics.

The reason for pancuronium bromide in the standard lethal injection method is not well understood. Judge Lyle found that Tennessee's method "was developed simply by copying the same method used in some 30 other states."

Deborah W. Denno, a law professor at Fordham University who published a study of the chemicals used in lethal injections last year, said that assembling information for it was difficult.

"The process has been so hidden," said Professor Denno, who described herself as an opponent of the death penalty.

Four states said their protocols for lethal injection were confidential; three said they had none. Except for New Jersey, which does not use pancuronium bromide, all 28 states that supplied Professor Denno with information used the same three-chemical combination. So does the federal government.

"The idea of even having a lethal injection protocol specifying exact chemicals is a very recent phenomenon," Professor Denno said, "and that's only because prison officials have been pressed to provide them. Nobody really knew what chemicals were being used."

The earliest protocol, in Oklahoma, was based on advice solicited by a state senator from a professor in the state's medical school. The professor, Stanley Deutsch, recommended an ultra-short-acting barbituate and a neuromuscular blocking drug like pancuronium bromide.

"I can assure you that this is a rapid, pleasant way of producing unconsciousness," Dr. Deutsch wrote in 1977.

In a recent interview, Dr. Deutsch stood by his initial finding, saying his method does not cause suffering. "They use such a massive amount of the penthothal that I don't think there is any chance that people will awaken," he said.

Other states, typically acting through their corrections departments and individual prison wardens, apparently copied Dr. Deutsch's advice without subjecting it to independent medical scrutiny.

The prison warden who developed Tennessee's method, for instance, testified that he did not consult with anyone with a medical or scientific background. The warden, Ricky J. Bell, declined to be interviewed.

A spokeswoman for the Tennessee attorney general also declined to comment and directed a reporter to the state's legal papers. They do not defend the use of pancuronium bromide beyond noting that it is widely used and so should not be considered cruel and unusual punishment under the Constitution.

The state's legal papers also argued that the ban on pancuronium bromide in pet euthanasia does not apply to Mr. Abdur'Rahman because he is not a "nonlivestock animal," which the law says includes pets, captured wildlife, exotic and domesticated animals, rabbits, chicks, ducks and potbellied pigs.

Carol Weihrer, who underwent eye surgery in 1998, testified for Mr. Abdur'Rahman at the hearing in May. Anesthesia was administered before the surgery, as was pancuronium bromide to immobilize the eye. But the anesthesia was ineffective. Ms. Weihrer testified that the experience was terrifying and torturous. She could not, she said, communicate that she was awake.

"I remember using every ounce of my strength to try to move," she said.

The surgery went on for hours. She did not suffocate because she was on a respirator.

Ms. Weihrer called the experience "worse than death."