Championship leader Charles Leclerc headed a Ferrari one-two from Red Bull's Max Verstappen in first practice at the Spanish Grand Prix.
Leclerc was 0.079 seconds quicker than team-mate Carlos Sainz, while Verstappen was 0.336secs off the pace but hit traffic on his fastest lap.
A number of teams have taken significant car upgrades to Barcelona.
So far there has been little effect on the order, with George Russell fourth ahead of Alpine's Fernando Alonso.
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Lewis Hamilton was sixth quickest, 0.221secs slower than team-mate Russell as Mercedes assessed a new floor package that they hope will provide some answers as to their car's poor performance so far this season.
Uncontrolled flexibility of the floor is at the heart of the "porpoising" or bouncing problem that has prevented Mercedes from accessing the performance they believe is in the car and if they can limit that they hope the car can make a step forward.
At the same time, comparing the data from this race with that from pre-season testing may allow Mercedes to draw some conclusions as to whether they need to rethink their entire design concept, which is different from any other car.
The biggest changes are to the Aston Martin, McLaren and Alfa Romeo cars and the one on the Aston has caused a stir as it appears to be a direct copy of the Red Bull.
Red Bull have questioned whether any of their intellectual property has made it on to the Aston Martin, especially as they have lost a number of staff to the team, but the FIA has investigated and ruled that all is above board.
Another off-track controversy surrounds Alonso, who has been referred to the stewards after heavily criticising the governing body.
McLaren's Lando Norris was seventh fastest, split from team-mate Daniel Ricciardo by the Alpha Tauri of Pierre Gasly, whose team-mate Yuki Tsunoda completed the top 10.
At the front, Ferrari have their first major upgrade of the season, the centrepiece is which is a new floor, while Red Bull have continued their programme of taking mass out of their car to try to get it down to the minimum weight limit.
It is said that a further 4kg - equivalent to about 0.14secs in a lap time - has been shed from the car since Verstappen won the Miami Grand Prix two weeks ago.