The US Supreme Court has blocked the execution of a Florida man to consider his appeal over the method used to carry out the punishment.
Hill argues the chemicals cause suffering that violates his rights
Clarence Hill was to be executed on Wednesday for the 1982 murder of a Pensacola police officer.
But the court wants to consider if the chemicals used in the execution cause pain - thus violating a Constitutional ban on cruel and unusual punishment.
Anti-death penalty campaigners say the move could delay other US executions.
Hill had been strapped to a gurney and intra-venous lines were running into his arms late on Tuesday night, his lawyer said, when Justice Anthony Kennedy issued a temporary stay.
The full court continued the stay on Wednesday.
"What a fantastic day! What a fantastic day," Hill's attorney D Todd Doss said, calling the ruling "a relief".
Oral arguments were set for 26 April, meaning it will likely be the summer before the court issues its opinion, Mr Doss told the Associated Press.
Lethal injection is the most widely used method of execution in the US, although some offer inmates the choice of alternatives like electrocution or the gas chamber.
METHODS OF EXECUTION
Lethal injection: Authorized in 37 states
Electrocution: In 10 states (sole method in Nebraska)
Gas chamber: In five states (all of which have lethal injection as alternative)
Hanging: Only in New Hampshire and Washington
Firing squad: In Idaho and Oklahoma
The exact combination of chemicals used varies, but the process usually involves three stages, according to the Death Penalty Information Center - an anti-death penalty group.
The condemned inmate is injected with an anaesthetic, which puts the inmate to sleep. Next flows pavulon or pancuronium bromide, which paralyses the entire muscle system and stops the inmate's breathing.
Finally, the flow of potassium chloride stops the heart.
Richard Dieter, head of DPIC, told the BBC: "The argument is that the chemicals may mask the pain of what is going on - we assume people are just put to sleep, but if you are paralysed it may mask the fact that these people are under extreme pain."
Mr Dieter added: "It is too early to say what bearing this case will have on the many other lethal injection executions scheduled, but it could potentially be very significant.
"It could hold up a lot of other cases and I think that is why the Supreme Court wants to expedite its consideration of this appeal."
No challenge to injection
The case allows the court to revisit a 2004 ruling in an Alabama death case, in which justices said that David Larry Nelson could pursue a last-ditch claim that his death by lethal injection would be unconstitutionally cruel because of his damaged veins.
While Hill does not have damaged veins, his appeal cites medical studies about the drug cocktail used by Florida and other states.
Mr Doss told the high court that there is a risk his client will not be fully anaesthetised at the time of his death and that courts should look at his latest claims.
Like the Nelson case, the new appeal is more about stopping an execution than a direct constitutional challenge to using the lethal injection.
Thirty-eight of the 50 US states and the federal government permit capital punishment.
In December, Kenneth Lee Boyd, a 57-year-old Vietnam veteran, became the 1,000th person to be executed in the US since the Supreme Court re-instated capital punishment in 1976.