Max Verstappen was fastest as Mick Schumacher made an eye-catching debut for Ferrari, finishing second on a rain-hit first day of testing in Bahrain.
Verstappen posted a 1:29.379 in the final 10 minutes of the day to finish six tenths ahead of Schumacher, who was making his first outing in a modern-day F1 car.
Schumacher - son of seven-time world champion Michael, who was a team-mate of Verstappen's father Jos in 1994 - racked up 56 laps and had moved to the head of the field with just over 20 minutes remaining before Verstappen's late flyer reclaimed top spot.
The 20-year-old will return in the Alfa Romeo on Wednesday.
The day was heavily disrupted by a four-hour rain delay in the afternoon but times tumbled once cars returned to track, although fuel levels and car set-ups are unknown.
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Lando Norris was restricted to just 22 laps due to the weather after taking over from Carlos Sainz in the afternoon but continued McLaren's encouraging start to the year by posting the third-best time.
Lewis Hamilton completed the most mileage for Mercedes, clocking 77 laps and posting the sixth quickest time of the day with a 1:31.156. Hamilton, who fortuitously won last Sunday's race after Charles Leclerc's late engine problem, had said Mercedes had a number of items to focus on.
The reigning world champion was behind Romain Grosjean's Haas, which had topped the timesheets before the rain delay, and Alexander Albon in the Toro Rosso.
Fernando Alonso, meanwhile, was 11th on his F1 return as he conducted a Pirelli tyre test in the second McLaren.
Daniel Ricciardo was seventh for Renault as the rain denied him the chance to get more familiar with his new car, ahead of Lance Stroll's Racing Point, Sainz, and Sauber's Antonio Giovinazzi.
George Russell was slowest for Williams, with the 21-year-old taking over the Mercedes from Hamilton on Wednesday.
*Compound 1 is the hardest tyre, Compound 5 is the softest
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