Ginsburg, in her dissent, said her colleagues should have asked Kentucky courts to consider whether the state includes adequate safeguards to ensure a prisoner is unconscious and thus unlikely to suffer severe pain.

Stevens, while agreeing with Wednesday's outcome, said the decision would not end the debate over lethal injection.

"I am now convinced that this case will generate debate not only about the constitutionality of the three-drug protocol, and specifically about the justification for the use of the paralytic agent, pancuronium bromide, but also about the justification for the death penalty itself," Stevens said in an opinion in which he said for the first time that he believes the death penalty is unconstitutional.

Stevens suggested that states could spare themselves legal costs and delays in executions by eliminating the use of the paralytic.

Kentucky has had only one execution by lethal injection, and it did not present any obvious problems, both sides in the case agreed.

But executions elsewhere, in Florida and Ohio, took much longer than usual, with strong indications that the prisoners suffered severe pain in the process. Workers had trouble inserting the intravenous lines that are used to deliver the drugs.

Roberts said "a condemned prisoner cannot successfully challenge a state's method of execution merely by showing a slightly or marginally safer alternative."

He acknowledged that Wednesday's outcome would not prevent states from adopting a method of execution they consider more humane. Alito and Kennedy joined his opinion.

Justice Stephen Breyer concurred in the outcome, although he said he would evaluate the case under the same standard put forth by Ginsburg, that a means of execution may not create an "untoward, readily avoidable risk of inflicting severe and unnecessary pain."

Scalia and Thomas said Roberts' opinion did not go far enough in insulating states from challenges to their lethal injection procedures, which were instituted to make executions more humane. A "method of execution violates the Eighth Amendment only if it is deliberately designed to inflict pain," Thomas said.

Donald Verrilli, a veteran death penalty lawyer who argued the inmates' case, said he was disappointed in the decision, but hopeful about other challenges. "I think important issues remain after this decision," Verrilli said. "Records of administration of lethal injections in other states raise grave doubts about the reliability of those procedures."

The Rev. Pat Delahanty, head of the Kentucky Coalition to Abolish the Death Penalty, said the ruling wasn't a surprise.

"We never expected it to do more than maybe slow down executions in Kentucky or elsewhere," Delahanty said. "We're going to be facing some executions soon."

Wednesday's case involved two inmates, Ralph Baze and Thomas Clyde Bowling Jr., who were convicted of murder and sentenced to death by juries in Kentucky. Baze killed a sheriff and a deputy who were attempting to arrest him. Bowling shot and killed a couple and wounded their 2-year-old son outside their dry-cleaning business.

Fayette County Commonwealth Attorney Ray Larson, who prosecuted Bowling in 1992, said after the ruling: "Fact of the matter is, this lethal injection process is about as far from cruel and unusual as anything you can imagine. This is just another one of those things the anti-death penalty gang is throwing against the wall to see what sticks."

The case is Baze v. Ress, 07-5439.

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E-mail: gfattah@desnews.com