Nigeria's ruling party presidential contender, Umaru Yar'Adua, 55, says he is well enough to run in April's polls.
Mr Yar'Adua says his check-up will only take a few days
Speaking to the BBC from Germany, where he was flown for a medical check-up, he scotched speculation that he was unfit to continue in the contest.
He said reports that he had collapsed and was rushed to hospital were "not true" and he would return home soon.
Mr Yar'Adua has been the frontrunner to win the elections and has previously suffered from a kidney illness.
The BBC's Alex Last says there has been increasing speculation that he could be replaced as the People Democratic Party's candidate.
He declared his candidacy after being backed by President Olusegun Obasanjo - who steps down in April after eight years in power.
Mr Yar'Adua said he had suffered from breathlessness and had taken his doctor's advice to have a thorough medical check-up abroad.
"I am only human, as they are," he said about his critics.
"I don't think a human being has control over his health of ill health, life or death and therefore they should not create conclusions on any other human being," he told the BBC's Focus on Africa programme.
His transfer to Germany led to a spate of rumours in Nigeria about the state of his health and the possibility that he could be replaced.
Not least, our reporter says, because being president of a country like Nigeria is such a gruelling task and presidential elections are only a month away.
"There are speculations all the time since the year 2000 surrounding my health," Mr Yar'Adua said.
In January, he sought to quash rumours about his health, inviting his critics to a game of squash if they could play 12 straight sets.
A tall, thin, quietly-spoken man, Mr Yar'Adua was a relatively unknown northern Muslim governor before he won the governing party primaries with the backing of President Obasanjo.
As the governing party's candidate, with access to its money and power, he is widely considered to be the favourite to become Nigeria's next president in elections in April.
Political gossips have suggested a number of alternatives should Mr Yar'Adua be replaced as the PDP's candidate, our correspondent says.
Gen Babangida is being touted as an alternative PDP candidate
Among them is the immensely wealthy former military ruler, Gen Ibrahim Babangida.
There are two other strong contenders in the dramatic race for the presidency.
Nigeria's Vice-President Atiku Abubakar is standing for the opposition Action Congress.
But he is currently involved in a bitter feud with the president and is battling on several fronts in the courts to clear corruption allegations which could bar him from running.
The other main opposition leader is former military leader Gen Muhammadu Buhari, who became famous for his war on corruption in the 1980s.
Despite being quite popular, critics have questioned whether he has the money and political machinery needed to win a Nigerian election.
The polls should become the first transfer of power in Africa's most populous country from one elected leader to another since independence in 1960.