Max Verstappen finished fastest during first practice in Barcelona, after George Russell complained of "bouncing" in his Mercedes in a repeat of their porpoising issues from last season.
Verstappen topped the timing sheets with a fastest lap time of one-minute 14.606secs, ahead of Sergio Perez and Esteban Ocon in second and third respectively.
However, the potential return of porpoising issues, particularly for Mercedes, overshadowed the session.
Russell took to the team radio early on during FP1 to complain of bouncing in the W14. Hearts will have sunk in the Mercedes garage as the message came through, with the team hopeful that the new upgrades introduced in Monaco might have come into their own this weekend.
However, Russell was not the only driver complaining of bouncing around the track, particularly heading into the final corner. Perez and Verstappen also struggled with the issue during FP1, with Lando Norris going as far to explicitly label the issue as "porpoising", not just bouncing.
Lewis Hamilton finished the session down in 12th - two places behind team-mate Russell in 10th - 1.239s behind Verstappen.
Nyck de Vries raised some eyebrows after he recorded the fourth fastest lap time of FP1, ahead of Pierre Gasly in fifth.
The teams will now have just a few hours to fine-tune and tweak their cars ahead of FP2, with Mercedes among those hoping that they can resolve the issue as quickly as possible.