Eddie Izzard has been ordered to rest during his attempt to complete 27 marathons in 27 days in South Africa.
The charity run - one for each year former South African president Nelson Mandela was imprisoned - is for Sport Relief.
The comedian and actor has so far completed four marathons in four days.
A BBC spokesman said: "After medical advice, Eddie visited a hospital in South Africa for tests.
"Eddie is in good spirits and is looking forward to continuing his marathon of marathons."
Damian Kavanagh, controller of BBC Three, said: "Eddie is determined to complete the challenge and we will be with him every step of the way on BBC Three."
The route Izzard is taking will trace Mandela's life story.
On February 23, the 54-year-old set off at 6.30am from Mbashe Bridge, near the town of Mvezo in the Eastern Cape of South Africa - the birthplace of the statesman.
The journey includes the school and university attended by Mandela, and then on to Cape Town and Robben Island - the place where he was imprisoned.
Along the way, Izzard is also visiting communities and organisations that have been helped by the work of Comic Relief.
The star's run will end at the Union Buildings in Pretoria where Mandela gave his inaugural speech as the first democratically-elected president of South Africa in 1994.
The revered leader died at the age of 95 on December 5, 2013.
In 2012 Izzard attempted a similar feat but had to pull out for health reasons.
The current trek follows his 2009 challenge which saw him complete 43 marathons in 51 days across the UK to raise more than £1.8 million.
Money raised from his latest venture - titled Eddie Izzard: Marathon Man - will help Sport Relief support disadvantaged people in the UK and across the world.
BBC Three's new online form will follow his progress with a daily personal diary and a virtual map.
:: A video diary is on YouTube under the heading Eddie Izzard - Marathon Man - Sport Relief.