Six British TV stars have set off from John O'Groats in a bid to cycle the length of Britain in a non-stop relay until they reach Land's End.
Fearne Cotton, David Walliams, Miranda Hart, Russell Howard, Patrick Kielty and Davina McCall are hoping to raise £1 million for Sport Relief with their pedal power.
They will be joined en route by a seventh rider - comedian Jimmy Carr.
The first 24 hours of the ride will sees an overall hill ascent of almost 29,000ft - the equivalent of Mount Everest.
Fearne Cotton says: "When I was told that we would be climbing the equivalent of Mount Everest on the first day alone, I thought it was a joke.
"But when you look at the route, it's true - there's almost 29,000ft for us to pedal up on the Scottish leg of the trip. And a lot of that is in the dark!"
Little Britain's David Walliams, who swam the English Channel for the same charity four years ago, said: "Water is one thing, but I'm not built for a bike. I'm not sure what I've let myself in for."
Click here to see how the stars prepared for the gruelling journey.