The court blocked Florida, at the last minute, from executing Clarence Hill in January, as Hill lay on a gurney with IV lines in his arms.
The justices took up his case with a lively and sometimes contentious discussion about the way states carry out capital punishment. The court's ruling will determine whether inmates can file last minute civil rights challenges claiming their deaths would be cruel and unusual punishment.
"Your procedure would be prohibited if applied to dogs and cats," Justice John Paul Stevens told Florida's assistant deputy attorney general, Carolyn Snurkowski.
On the other side, Justice Antonin Scalia said the Constitution does not require painless deaths. "Hanging was not a quick and easy way to go," he told Hill's lawyer, referring to one of the country's oldest execution methods.
States gradually have stopped using hangings, firing squads, gas chambers and electric chairs. Now the federal government and every capital punishment state but one uses lethal injection because it is considered more humane. Nebraska still has the electric chair, but its use is being challenged in court.
Critics of lethal injection have been bolstered by a 2005 study published in the Lancet medical journal indicating that a painkiller administered at the start of an execution can wear off before a prisoner dies.
Hill's lawyer, D. Todd Doss, said Hill accepts that he can be executed for slaying police officer Stephen Taylor in the coastal town of Pensacola 24 years ago. Hill just does not want to suffer, Doss said.
Florida argues that it is too late for Hill to contest the plans for his death. Snurkowski said the only way Hill could file a challenge to lethal injection is if Hill comes up with an alternative proposal. That argument angered several court members.
Justice David H. Souter said "why does he have an obligation ... to tell the state how to execute people?"
"Doesn't the state have a minimal obligation on its own" to investigate whether its executions cause gratuitous pain, asked Justice Anthony M. Kennedy.
Later, Kennedy reprimanded his colleagues for laughing as several justices joked about the mischief that defense lawyers could cause if forced to propose ways to execute their clients.
"This is a death case," snapped Kennedy, who is expected to be a key vote in the case.
Chief Justice John Roberts said that death row lawyers, if allowed to pursue last-minute challenges, could drag out appeals.
Florida's three-drug combination is similar to that used in other states. The painkiller sodium pentothal is followed by a chemical, pancuronium bromide, that paralyzes the inmate. The final drug is potassium chloride, which causes a fatal heart attack.
Florida is one of 30 states that restrict the use of an agent such as pancuronium bromide in euthanizing animals, justices were told in a brief by three veterinarians. The veterinarians said "its only effect is to mask any suffering endured by the patient." They said that the Florida protocol does not meet standards for animals.
Several justices appeared surprised that the state has laws that spell out how animals should be euthanized, but that there are no guidelines for how prison officials should execute people.
Snurkowski said the protocol was devised by prison officials six years ago after the state stopped using its electric chair, nicknamed "Old Sparky," unless an inmate specifically requests death by electrocution.
Justice Stephen Breyer said it "doesn't seem too difficult" to alter the drugs because of concerns and that the state should not "have any interest in causing pain."
The court's ruling, which will be announced before July, will deal with a limited part of the subject: whether inmates can file special last-minute civil right challenges to the chemicals used in lethal injection even if inmates have exhausted all their regular appeals.
The justices' decision to hear the case renewed legal efforts around the country on behalf of death row prisoners; executions have been blocked in California, Maryland and Missouri.