Pascal Wehrlein will swap from Force India to Mercedes for the afternoon session at Barcelona as Lewis Hamilton has taken ill.
The reigning World Champion was in action for Mercedes at the start of Thursday's running but, after two hours in the car, was deemed to be too unwell to continue.
Mercedes announced that their test driver Wehrlein, who was at the track with Force India, would therefore swap over to the W06 for the afternoon.
"An unplanned stoppage keeping us off track at the moment," the team tweeted. "Lewis Hamilton is unwell and not able to continue driving today.
"We're just adjusting the car for Pascal Wehrlein to take to the track this afternoon."
Mercedes added: "We've borrowed him back from our friends Force India F1. Thanks guys!"