Peter Robinson has wished his DUP colleague Nigel Dodds a speedy recovery and said he "hopes to be discharged in the morning".
The North Belfast MP was in the party's offices at the House of Commons when the episode unfolded just before Prime Minister's questions this morning.
He was taken to a nearby hospital.
It is understood Mr Dodds (54) is undergoing a series of tests.
Mr Dodds - a former finance minister in the Northern Ireland Executive - is married to sitting DUP MEP Diane Dodds.
A Cambridge graduate, he has been a Member of Parliament for North Belfast since 2001.
DUP leader Peter Robinson said he hoped his party colleague would have a speedy recovery.
"I have spoken with Diane this evening, Nigel is in good spirits and is feeling much better.
"He will be staying in hospital overnight for observation but hopes to be discharged in the morning.
"Everyone in the party wishes him a full and speedy recovery. Nigel and Diane would appreciate some privacy and have no plans to make any further comment at this time," Mr Robinson said.