LOS ANGELES — In Tennessee, it is a crime to euthanize a cat with pancuronium bromide, but this doesn't stop the state from using it to execute condemned criminals. Because the drug paralyzes muscles but does not affect nerves, it may leave its victims wide awake but immobilized as they painfully suffocate. So prisoners' advocates and medical experts are now trying to persuade Tennessee — and the nearly 30 other states that use the drug — to choose different poisons for lethal injection, thereby bringing euthanasia protocols for humans in line with those for domestic animals.

And so continues the uniquely American habit of tinkering with the machinery of death. For the past century and a half, America's capital punishment debate has resembled a strange game of leapfrog: opponents of the death penalty claim that the current method, whatever it may be, is barbaric, which prompts capital-punishment supporters to refine that method or develop a new one.

Although 19th-century Americans tended to believe that justice and order demanded the ultimate sanction, they were often shaken by graphic accounts of the pain suffered by hanged men. In 1876, after an especially gruesome hanging, Maine abolished capital punishment. Inspired by this victory, opponents of the death penalty began to emphasize the cruelty of the gallows. But their effort was self-defeating: by claiming that the problem with hanging was the suffering of the condemned, they simply challenged death penalty advocates to find a better way to kill.

First came adjustments to the gallows. Hangmen created a formula in which rope length was a function of the prisoner's weight — the heavier the victim, the shorter the drop. But such delicate calculations of anatomy and gravity often failed to add up, and many prisoners slowly strangled to death. To dull the pain, Brooklyn officials in 1847 knocked a murderer cold with ether before hanging him, but this simply highlighted the deficiencies of the gallows.

Then, in 1889, New York State built the first electric chair, a device championed by Thomas Edison. Edison's advocacy was inspired in part by a wicked plan to hurt his business rival, George Westinghouse — the chair was powered by Westinghouse's alternating current, and Edison hoped consumers would begin to associate AC with danger and death. But Edison had less cynical reasons as well: he was an opponent of the death penalty — "an act of foolish barbarity," he called it — and he believed that electrocution would be less barbaric than the noose. Many others agreed, and eventually 25 states and the District of Columbia installed electric chairs.

Electrocution remained the state of the art for three decades, until the public grew dismayed by bungled executions that required several shocks or set the prisoner on fire. Before long there was another scientific option: an airtight chamber filled with poison gas, adopted by Nevada in 1924 and then by 10 more states in the coming decades. Like all complex machines, however, these execution devices were prone to malfunction, and prisoners suffered the consequences.

So in 1977 Oklahoma began to poison condemned prisoners with a three-drug cocktail: sodium thiopental (to produce unconsciousness), pancuronium bromide (to paralyze the muscles) and potassium chloride (to stop the heart). Promising a clean, painless death, this protocol quickly gained widespread acceptance.

Until now, that is. The next step seems obvious: states will adopt a different drug regimen, which, no doubt, will soon gain critics of its own.

However, it seems that many death-penalty opponents are realizing that technological leapfrog is a game they can't win, and are opting out of the latest debate. Amnesty International has issued this statement: "The search for a `humane' way of killing people should be seen for what it is — a search to make executions more palatable." In Nebraska, the only state with the electric chair as its sole method of execution, State Senator Ernie Chambers has vowed to fight any attempt to "make executions easier." He hopes that the United States Supreme Court will one day declare electrocution unconstitutional, leaving Nebraska without a valid execution law.

Some of the condemned themselves have even sought a more painful death in order to highlight the hypocrisy of "painless execution." In 2001, John Byrd, a convicted murderer in Ohio, requested electrocution rather than the needle; when prison workers balked at using the chair, which had been idle for nearly 40 years, the Legislature abolished electrocution and forced Mr. Byrd to die by lethal injection. Earl Bramblett, a Virginia prisoner, had more success in his protest. "I'm not going to lay down on a gurney and have them stick a needle in my arm and make it look like an antiseptic execution," he said, and he died in the electric chair on April 9.

For too long, defenders of capital punishment, fearing that brutal killing methods might provoke public opposition, have found unwitting allies among their adversaries, anxious to relieve the suffering of the condemned. Now death penalty opponents are realizing that scientific execution methods, ceaselessly refined, simply mask the barbarity of killing.

Mark Essig is author of "Edison and the Electric Chair: The Story of Light and Death."