''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.