Felipe Massa led Fernando Alonso in a Ferrari one-two in first practice at the Bahrain Grand Prix.
Massa edged Alonso by just 0.077secs as Mercedes driver Nico Rosberg was third ahead of Red Bull's Sebastian Vettel.
Force India's Paul di Resta was fifth from McLaren's Jenson Button and Red Bull's Mark Webber.
Di Resta's team-mate Adrian Sutil was eighth ahead of Lotus drivers Kimi Raikkonen and Romain Grosjean. Mercedes' Lewis Hamilton was 13th.
The practice session suggested competition would be close at the front - Rosberg was just 0.134secs behind Massa and 0.169secs ahead of Vettel.
Button's team-mate Sergio Perez, under pressure to match the Englishman, was 11th fastest, 0.571secs behind.
BBC F1 technical analyst Gary Anderson said: "Ferrari have what you might call a car for all seasons. It looks like they can put in quick laps fairly early in the session, which gives them a good baseline.
"I was expecting more from Red Bull. This circuit should suit their car and I expect them to show more through the weekend. Raikkonen only does what he needs to do at any one time.
"It's good to see Jenson Button up there - I saw more smiles from team boss Martin Whitmarsh than I have done recently."
Raikkonen, second in the championship after a strong start to the season, spent a considerable time in the garage as the Lotus team changed his rear suspension settings.
Many of the top teams were trying out updates to their cars.
Red Bull had a new wheel design, featuring a hole through the hub to enhance aerodynamics. Only Vettel had it on his car.
At the back, the Caterhams were ahead of the Marussias for the first time as Frenchman Jules Bianchi, one of the stars of the season so far, sat out the first session for Marussia in favour of Venezuelan reserve driver Rodolfo Gonzalez.
Caterham had their former race driver Heikki Kovalainen in Giedo van der Garde's car for the session, in his new role as reserve driver.
Using their basic car to assess its condition, the Finn was 0.551secs off Frenchman Charles Pic in an updated model.
During the session, governing body the FIA issued a joint statement with the sport's commercial arm Formula 1 Management (FOM) to say that they believed the race "should go ahead" despite ongoing civil unrest in the Gulf state.
1. Felipe Massa (Brz) Ferrari 1min 34.487secs
2. Fernando Alonso (Spa) Ferrari 1:34.564
3. Nico Rosberg (Ger) Mercedes 1:34.621
4. Sebastian Vettel (Ger) Red Bull 1:34.790
5. Paul di Resta (GB) Force India 1:34.949
6. Jenson Button (GB) McLaren 1:35.069
7. Mark Webber (Aus) Red Bull 1:35.101
8. Adrian Sutil (Ger) Force India 1:35.119
9. Kimi Raikkonen (Fin) Lotus 1:35.345
10. Romain Grosjean (Fra) Lotus 1:35.611
11. Sergio Perez (Mex) McLaren 1:35.640
12. Valtteri Bottas (Fin) Williams 1:35.783
13. Lewis Hamilton (GB) Mercedes 1:35.792
14. Jean-Eric Vergne (Fra) Toro Rosso 1:36.014
15. Daniel Ricciardo (Aus) Toro Rosso 1:36.485
16. Pastor Maldonado (Ven) Williams 1:36.498
17. Nico Hulkenberg (Ger) Sauber 1:36.755
18. Esteban Gutierrez (Mex) Sauber 1:37.214
19. Charles Pic (Fra) Caterham 1:37.850
20. Heikki Kovalainen (Fin) Caterham 1:38.401
21. Max Chilton (GB) Marussia 1:39.445
22. Rodolfo Gonzalez (Ven) Marussia 1:40.215