Mercedes driver Lewis Hamilton set the pace in first practice at the Spanish Grand Prix with McLaren's Jenson Button a surprise second.
Hamilton was 0.868 seconds clear of Button as Mercedes continued their domination of Formula 1.
Red Bull's Daniel Ricciardo was third, 0.082secs behind Button, with Ferrari's Fernando Alonso fourth.
Hamilton's team-mate Nico Rosberg was fifth fastest after a troubled session in which he managed just nine laps.
The German was stopped by overheating in his car's hybrid system, but co-team boss Toto Wolff said the car would be ready in time to go out in the second session.
Red Bull team principal Christian Horner said he hoped his car would have closed the gap to Mercedes after the introduction of the latest upgrades up and down the pit lane.
But on the face of it, Mercedes' advantage appeared as big as ever. Hamilton did report, however, that his "rear tyres were taking a beating" on a track where he plummeted down through the field with tyre wear problems last year.
Ferrari's Kimi Raikkonen was sixth fastest, 0.209secs slower than Alonso after a trouble-free session.
The Ferrari drivers were comparing two front-nose designs, one with more downforce than the other, to see which one was more effective on this track that makes great demands of the aerodynamic efficiency of the car.
Alonso was in spectacular form, visibly pushing the car to the very limit as he evaluated the new parts under the gaze of president Luca Di Montezemolo, who arrived at the track halfway through the session.
Di Montezemolo is attending a race for the second time this season. He was also in Bahrain, where Alonso and Raikkonen finished ninth and 10th, leading to the resignation of team principal Stefano Domenicali.
He has been replaced by Fiat protege Marco Mattiacci, who is attending his second race in the position this weekend, while Di Montezemolo has said he will take a more hands-on role with the team as they struggle to get back to the front in F1.
McLaren's Kevin Magnussen was seventh, 0.523secs slower than Button, with Lotus' Pastor Maldonado, Force India's Sergio Perez and Williams' Felipe Massa 10th.
The second Red Bull driver Sebastian Vettel was only 20th overall after stopping out on track with an electrical problem after just four laps.
Horner admitted that realistically Red Bull and Ferrari would be scrapping for third place on the podium behind Hamilton and Rosberg, as they were in China, where Alonso claimed the position.
Perez suffered an unusual problem when his left rear-view mirror came loose and flapped about, held on only by a retaining strap.
Both Sauber drivers had an eventful session. Using a new chassis which brings the team closer to the minimum weight limit after racing with an overweight car for the first four races of the season, Adrian Sutil and reserve driver Giedo Van Der Garde both had offs.
Sutil lost control at the high-speed uphill Turn Nine, running wide and dropping a wheel into the gravel.
Van Der Garde's incident was more concerning. He went straight on at Turn One, afterwards reporting a brake failure.