The top court in Arkansas has halted two executions that were due to start a series of seven in 11 days.
The state's Supreme Court acted hours before convicted murderers Don Davis and Bruce Ward were due to die, but the Davis decision could yet be reversed.
Their lawyers had argued they were mentally unfit to face execution and were denied proper assessment.
But there was a victory for the state when a federal court lifted an order that blocked all seven executions.
A blanket ruling made on Saturday that the executions could not go ahead on the grounds that the lethal injection causes suffering has been overruled.
The unprecedented pace at which Arkansas plans to put the seven men to death has drawn international attention.
It is driven by the state's desire to use a batch of drugs before it expires later this month.
But it has been condemned by critics as an inhumane "assembly line".
What happened on Monday?
There were two rulings - in the first, the Arkansas Supreme Court stayed the executions of Davis and Ward, in a 4-3 decision. They have each spent more than 20 years on death row.
The state attorney general's office said it would not appeal against the stay for Ward. It did appeal against the stay for Davis but the US Supreme Court upheld it in a last-minute decision on Monday.
Davis had already been served his last meal and moved to the Cummins Unit, which holds the lethal injection gurney.
In the other decision, the 8th US Circuit Court of Appeals in St Louis cancelled the weekend ruling to halt all seven executions.
What did all these men do?
Bruce Ward - Strangled teenage shop clerk Rebecca Doss
Don Davis - Condemned for the execution-style killing of Jane Daniel as he burgled her home
Stacey Johnson - Murdered Carol Heath, who was beaten, strangled and had her throat slit
Ledell Lee - Bludgeoned Debra Reese to death with a tyre iron her husband had given her for protection
Jack Jones - Condemned for the rape and murder of accounts clerk Mary Phillips, and the nearly fatal beating of her 11-year-old daughter
Marcel Williams - Raped and murdered Stacey Erickson, after kidnapping her from a convenience store
Kenneth Williams - Murdered farmer Cecil Boren during an escape from prison where Williams had been incarcerated for murdering cheerleader Dominique Hurd
Why is there so much uncertainty?
Like many US states, Arkansas has struggled to find the drugs it needs to carry out executions. Its last was in 2005.
The state's use of midazolam in its three-drug cocktail is controversial, because opponents of the death penalty say it is not effective at rendering the inmate unconscious.
The drug raised concerns after it was used in executions in three US states in 2014 that took longer than usual.
There are five more executions in Arkansas scheduled to happen by the end of April, so expect daily legal arguments on both sides.
The state is determined these executions be carried out, but those who have campaigned against the methods of execution will do all they can to block them.