The court ordered Florida to stop the evening execution of Arthur Rutherford, who claims that the state's lethal injection procedure is cruel and unusual punishment.
The court's action came just as Florida was preparing to execute Rutherford and only a few hours after justices gathered privately for the swearing-in of new Justice Samuel Alito. Alito did not consider Rutherford's case.
Florida inmate Clarence Hill, after he had already been strapped to the gurney. Justices said they would use Hill's case to clarify how inmates may bring last-minute challenges to the way they will be put to death. Arguments are expected this spring.
The intervention in Rutherford's case was not a surprise because he also brought a late challenge to lethal injection.
An appeals court said he could not pursue the claim.
Rutherford's lawyer, Linda McDermott, said that he was entitled to a chance to prove that the chemical cocktail used in Florida's lethal injection process "constitutes deliberate indifference to a foreseeable risk of the gratuitous and unnecessary infliction of pain."
The stay Tuesday will remain in place until justices deal with the issues raised by Hill and Rutherford. That probably will be before July.
Rutherford, 56, was convicted of robbing and killing Stella Salamon, a 63-year-old widow originally from Australia, in 1985 at her Milton, Fla., home, where the handyman had installed some sliding glass doors.
The woman was severely beaten and drowned or asphyxiated. Her body was found in a bathtub. Four witnesses testified that Rutherford either told them that he was planning to kill her or admitted it afterward.
Hill was to be the 61st inmate executed in Florida since 1976, when executions resumed after a 12-year moratorium.
In one of his appeals, Hill, 48, asked for a delay to give him time to contest the chemicals that will be used. Kennedy cited that case in granting the stay.
Hill's lawyers argue that the three chemicals used in Florida's lethal injection method cause pain, making his execution cruel and unusual punishment.