Kenya's Prime Minister Raila Odinga has had an operation to remove fluid on the brain, doctors say, after initial reports he had been hospitalised for exhaustion.
Neurosurgeon Oluoch Olunya, who carried out the operation, said he was recovering in the Nairobi Hospital.
"I'd like to assure the country he is actually well," he said.
The BBC's Peter Greste in Nairobi says Mr Odinga, 65, has been one of Kenya's most energetic politicians.
In recent months, he has been campaigning particularly hard to win support for a new constitution, with no questions about his physical fitness.
So our correspondent says Tuesday's statement from his press spokesman came as something of a surprise.
He had said Mr Odinga, was fit to work but had been advised to rest.
Mr Olunya said the prime minister had checked himself into hospital on Monday afternoon, complaining of a headache and fatigue.
He said there had been a build-up of pressure, which was relieved when some fluid was removed through a small opening in the skull.
Mr Olunya said the pressure might have been caused by him hitting his head while in a car three weeks ago.
"He is sitting comfortably in his room," he told journalists, adding that the prime minister should be fit to leave hospital in five days' time.
Until now, most of the questions about the health of Kenya's political leaders have been directed at President Mwai Kibaki, our correspondent says.
He disappeared from public view shortly after he was first elected in 2002, and rumours persisted that he had suffered a stroke.
His staff, however, consistently denied he had suffered anything beyond minor health issues.
Mr Odinga has been prime minister since 2008 under a deal brokered to end months of violence after his supporters claimed he had been cheated of victory by allies of President Kibaki.
The coalition government remains shaky but both men are campaigning in favour of the new constitution ahead of a referendum due in August.