Supreme Court justices appear divided over whether the method of lethal injection that is used in most states is permissible under the Constitution.

Under the procedure that came before the court Monday, three drugs are administered in succession to knock out, paralyze and kill prisoners. Lawyers argue if the initial anesthetic doesn't take hold, the third drug could cause serious pain.

Justice John Paul Stevens says he's "terribly troubled" that the method could result in excruciating pain.

But fellow justice Antonin Scalia is suggesting he'll uphold Kentucky's method of capital punishment, saying, "there is no painless requirement" in the Constitution.

A veteran Washington lawyer says the risk is real, and argues that it's "unlawful to euthanize animals the way Kentucky executes inmates." Kentucky bars the use of the paralytic on animals.

Kentucky, backed by the Bush administration, says it works hard to execute inmates humanely. It contends the procedure remains within the confines of the 8th Amendment ban on cruel and unusual punishment.

Recent executions in Florida and Ohio took much longer than usual, with strong indications that the prisoners suffered severe pain.