Pascal Wehrlein will drive for Mercedes during today's second day of in-season testing at the Circuit de Barcelona-Catalunya.
Wehrlein ran for his regular race team, Manor, on the opening day of the test and was not set to run today.
Mercedes was set to give track time to its young gun Esteban Ocon, who received a run out for Renault on Tuesday.
Mercedes says the move was prompted by the need to use an experienced driver to test a few items left over from Tuesday's running.
Ocon will now make his Mercedes debut during the second in-season test at Silverstone in July.
Mercedes will now have to run young drivers on both days of the in-season test, with 50% of track time across the course of the four in-season test days obliged to be devoted to drivers with fewer than two Grand Prix starts.