World champion Casey Stoner set the pace in the first practice for the season-opening Grand Prix of Qatar.
The Honda rider clocked 1min 56.474secs to edge out former champion Jorge Lorenzo's Yamaha Factory Racing bike.
Ducati's Nicky Hayden was almost half a second slower than Stoner in third.
British rider Cal Crutchlow was sixth fastest, while seven-time champion Valentino Rossi, who has won this race three times, was 10th, more than a second behind Stoner.
Crutchlow, 26, ended his debut season last year with his best performance - a fourth place finish in the final race. The Tech 3 Yamaha rider has been backed to challenge for top five finishes this season by his team.
"That's not a bad way to start 2012 at all and I'm pretty happy to be in the top six," Crutchlow said. "It is a good platform to build on for the rest of the weekend and now I'm really looking forward to the race on Sunday night."
Qualifying takes place on Saturday after further practice sessions. Stoner will start as favourite for Sunday's race, which will be held under the Losail circuit floodlights, having won four times in the elite class in six visits.
The world champion said: "It was an OK start tonight. We finished quickest, which is good, but we'll have to wait until tomorrow for some real track time and with the conditions a little better and see where we are."
Lorenzo said he felt good after his second-fastest time: "We are in the top close to Casey and we have improvements to come tomorrow from the bike, the track and myself."
In Moto3, the new lowest cyclinder class, Danny Kent came second, while fellow Brit Scott Redding was fifth in Moto2.
FIRST PRACTICE MOTOGP QATAR, FASTEST TIMES
1 Casey Stoner (AUS), Repsol Honda 1:56.474
2 Jorge Lorenzo (SPA) Yamaha +0.174
3 Nicky Hayden (USA) Ducati +0.450
4 Ben Spies (USA) Yamaha +0.508
5 Dani Pedrosa (SPA) Repsol Honda +0.656
6 Cal Crutchlow (GB) Tech 3 Yamaha +0.921
7 Alvaro Bautista (SPA) Gresini Honda +1.038
8 Andrea Dovizioso (ITA) Tech 3 Yamaha +1.073
9. Hector Barbera (SPA) Pramac Ducati +1.438
10 Valentino Rossi (ITA) Ducati +1.440