Ferrari's Fernando Alonso beat Red Bull's Sebastian Vettel to set the fastest time in first practice at the Spanish Grand Prix.
The Spaniard, trying out a series of new parts on his updated car, was 0.378 seconds faster than the world champion.
Sauber's Kamui Kobayashi was third, ahead of McLaren's Jenson Button, whose team-mate Lewis Hamilton was eighth.
Williams driver Valtteri Bottas was fifth ahead of Michael Schumacher's Mercedes and Romain Grosjean's Lotus.
Most of the teams spent the session analysing the new parts they are introducing for this first European race of the season.
A lot of attention is on Ferrari, who need to quickly improve their car if Alonso is to have any chance of battling for the championship.
He has performed superbly in an uncompetitive car to lose only 10 points to championship leader Vettel after the first four races, but has admitted Ferrari need to start making major steps forward.
Although Alonso has played down expectations, the car has been fitted with new rear bodywork, a new floor and new front and rear wings.