DUP MLA William Hay is undergoing tests in Altnagelvin hospital after complaining of chest pains.
The Speaker of the Assembly was due to chair tribute to former First Minister Ian Paisley at Stormont but had to miss the occasion.
He was admitted to hospital on Sunday after complaining of chest pains during the weekend.
A statement from the Assembly said: “Like many members expressed in the Chamber today, we wish the Speaker a speedy recovery and look forward to seeing him back in Parliament Buildings.”
Mr Hay could not be reached, although one of those going to visit him was DUP colleague, Gregory Campbell.
Mr Campbell said: “I know Willie was extremely disappointed that he didn’t get an opportunity to take the chair for this event. Like myself, he’s known Ian Paisley for about 40 years.”
He said he was not seriously ill, and expected it would be this afternoon at the earliest before anything was known about a return to work for the 64-year-old.
In the meantime, the Assembly said Mr Hay has authorised his deputies to exercise the functions of the Speaker’s Office on his behalf.
Chairing the session yesterday instead was Sinn Fein MLA and Principal Deputy Speaker Mitchel McLaughlin.