Mercedes driver George Russell set the pace from McLaren's Oscar Piastri in first practice at the Barcelona-Catalunya Grand Prix.
Russell was 0.203 seconds quicker than Piastri, with Ferrari's Charles Leclerc third ahead of Red Bull's Max Verstappen.
The session was marked by six junior drivers replacing the regulars as part of the mandatory requirement for teams to use at least two practice sessions a year for up and coming drivers.
McLaren's Leonardo Fornaroli was the fastest, in fifth place, ahead of Estonian Paul Aron in an Audi, on loan from his regular role as Alpine reserve driver.
Racing Bulls' Liam Lawson was seventh, separated from team-mate Arvid Lindblad by Dino Beganovic, driving Lewis Hamilton's Ferrari.
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Alpine's Franco Colapinto completed the top 10, ahead of Haas driver Oliver Bearman and Audi's Gabriel Bortoleto.
The session was relatively incident-free, although Cadillac's reserve driver Colton Herta had a moment over the kerbs at Turn Eight, as did Alpine's Pierre Gasly and Beganovic.
Leclerc was testing a major aerodynamic upgrade on the Ferrari, including a new floor, which Hamilton will also use in the second session this afternoon.
Briton Luke Browning, Williams' reserve driver, managed no laps as a result of an electrical problem in Alex Albon's car.
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