The Supreme Court refused Monday to directly consider whether the drug combination used in executions across the country amounts to unconstitutional cruel punishment.
The justices had already agreed to hear arguments in April in a case brought by Florida death row inmate Clarence Hill about the procedure for lethal injection challenges to be filed in federal court.
Monday's decision, which came on a separate appeal by Hill's lawyer, has little practical significance because Hill's other case is still pending.
The justices did not comment when they declined to broaden their review and take up Hill's appeal that raised constitutional concerns about lethal injection.
The execution method is used by the federal government and every state that has capital punishment, except for Nebraska.
Interest in lethal injection has escalated in recent weeks. In California, an execution was postponed last week when no doctor or nurse would agree to administer a fatal dose of barbiturate. A judge is reviewing the state's system.
Death penalty supporters contend that the Constitution does not guarantee convicted killers a pain-free execution.
Hill is on death row for killing a Pensacola, Fla., police officer after a bank robbery in 1982.
Hill came within minutes of being executed last month when the court intervened. His lawyer said he was already strapped to a gurney when the high court issued a stay.
His lawyer said that the doses of drugs used in Florida executions sodium pentothal, pancuronium bromide and potassium chloride can cause pain. The first drug is a painkiller. The second one paralyzes and the third causes a fatal heart attack.
The attorney, D. Todd Doss of Lake City, Fla., pointed to a 2005 study published in the Lancet medical journal indicating that the painkiller can wear off before an inmate dies. The Florida Supreme Court ruled that the study was inconclusive.
The latest case is Hill v. Florida, 05-8731.
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