Peter Robinson is facing an uncertain political future after he spent last night in hospital following a heart attack.
The Northern Ireland First Minister (66) was rushed to hospital yesterday morning with chest pains and underwent a procedure later in the day.
The DUP leader recently took up cycling to improve his health, but had to stop twice during a ride with party colleague Sammy Douglas at the weekend, complaining of tiredness.
Mr Robinson felt unwell early yesterday morning. An ambulance came to his house and took him to the Ulster Hospital, which is close to his home in the Castlereagh hills.
He was later moved to the Royal Victoria Hospital where heart procedures, such as the insertion of stents, are performed.
The health scare came on the eve of a crunch Stormont debate on welfare reform which is expected to proceed without Mr Robinson today.
The party has given no indication of who will assume the leadership if Mr Robinson is absent for some time, but Arlene Foster has adopted the role before.
Mr Robinson's family are waiting to hear the prognosis and have asked, through the DUP, for their privacy to be respected by the media.
Mr Douglas, a fellow East Belfast MLA, said he and Mr Robinson had been cycling together for the past two Saturdays.
He said last Saturday Mr Robinson was in good form, but he noticed the First Minister was more tired than the week before.
"He had to stop twice; I thought at the time he was just tired and he said he had a spot of indigestion, soreness around the chest or stomach it sounded like. But he was in the best of humour," he said.
Deputy First Minister Martin McGuinness said his thoughts and prayers were with Mr Robinson and his family.
Mr Robinson, a father of three, has been leader of the DUP and First Minister since 2008. As leader of Stormont's largest party, Mr Robinson has been under significant pressure in recent months, not least because of the welfare reform wrangle with Sinn Féin which has threatened to take down the Northern Ireland Executive.
The DUP also endured a difficult Westminster general election campaign when former health minister Jim Wells resigned after a furore about comments he made linking same-sex marriage to child abuse.