The US Supreme Court has started hearing arguments on whether to ban executions by lethal injection.
The standard method is a fatal combination of three chemicals
Justices have argued for and against the issue, raised by the challenges from two Kentucky death row inmates - Ralph Baze and Thomas Clyde Bowling Jr.
They sued the state in 2004, saying three-drug injections violated the US Constitution's ban on cruel punishment.
The court's decision to hear the case last September has halted executions across the country.
Lethal injection is used in all the 37 states that have capital punishment except Nebraska, which requires electrocution.
Sodium pentothal - anaesthetic
Pancuronium bromide - paralyses entire muscle system
Potassium chloride - stops the heart
The method used by most states is a combination of three chemical injections - one which makes the inmate unconscious, another that paralyses all muscles except the heart, and a final drug that stops the heart, causing death.
Opponents say that if one of the chemicals fails, the prisoner will suffer excruciating pain.
Justice Antonin Scalia was among several conservatives in court who supported Kentucky's choice of method.
"There will always be some claim that there is a new and better method of execution, and once again all executions will be stalled," he said.
He warned that delaying a decision could mean a long lasting halt on executions, adding "we wouldn't want that to happen".
Other justices were concerned by the three-drug method. Justice John Paul Stevens said lawyers for Kentucky had given a good account of how safeguards were taken.
"But I'm terribly troubled by the fact that the second drug could cause excruciating pain," he said.
In 2004, Baze and Bowling - who had both been convicted of murder and sentenced to death - argued that lethal injections administered in Kentucky amounted to cruel and unusual punishment, violating the Eighth Amendment of the US Constitution.
The procedures "create a significant and unnecessary risk of inflicting severe pain that could be prevented by the adoption of reasonable safeguards", their lawyers said in court papers.
The Kentucky state defends its procedures.
"Kentucky seeks to execute in a relatively humane manner and has worked hard to adopt such a procedure," Kentucky Attorney General Gregory Stumbo has said.
A ruling by the Supreme Court is expected in June.
Lawyers for the two men have said the court has not reviewed the issue for more than 100 years. In 2007, 42 people were put to death in the US - the lowest number in 13 years.
In December, New Jersey became the first state to abolish the death penalty since 1976.