The party stalwart was treated for a heart condition last year and was taken into the Ulster Hospital, near Belfast at the weekend.
"He is in hospital for tests but he is in good spirits and his family has asked for privacy," a spokeswoman said.
Mr Paisley, 87, known as Lord Bannside, led the DUP into power-sharing at Stormont with Sinn Fein in 2007.
In February 2012, the 87-year-old spent a week on a life support machine suffering from heart failure.
The previous year he had a pacemaker fitted at St Thomas' hospital in London after falling ill at Westminster. Paramedics had to revive him following his collapse in parliament.
Mr Paisley stood down as Northern Ireland's first minister in 2008 and ended 60 years of full-time ministry in January 2012.
The veteran unionist and fundamentalist Protestant preacher has been a colossus of Northern Ireland politics.
He was an MP for North Antrim and a divisive figure at a difficult time.
Mr Paisley established the DUP in 1971 and opposed a power-sharing government between nationalists and unionists in Northern Ireland. But in a dramatic change of heart after the St Andrews agreement in 2006 he indicated that the DUP would share power with their former enemies in Sinn Fein.