Listeners were left baffled by the erratic behaviour of Radio 2 presenter Ken Bruce - until it emerged the show was being hosted by comedian Rob Brydon.
Rob managed to get through almost the entire programme before eventually dropping hints that it was an April 1 wind-up.
Listeners took to message boards wondering whether Ken was drunk.
Funnyman Rob had still not owned up by the end of the show when he handed over to the next presenter Jeremy Vine, who he called Tim - the name of Vine's comedian brother.
Some listeners were fooled by the switch. One tweeted: "Ken Bruce has been really annoying this morning totally unlike him but I for one am not impressed. There, I've said it now, rant over."
Others saw through the gag and praised Rob's appearance.
And the comic himself dropped a hint during the show, when he posted a message on Twitter which said: "Loving Ken Bruce on Radio 2 this morning..."