Marco Simoncelli was quickest as the Assen weekend began with a very wet first free practice session.
The Gresini Honda rider hit the front with a 1m49.036s two minutes from the end. He had spent the early part of the morning exchanging the top spot with Valentino Rossi, and after being pushed down the order slightly, the former champion jumped back into second, 0.3sec down on Simoncelli, with his final lap.
This session was Rossi's first chance to try the 2012 Ducati chassis with the 800cc engine, although the wet conditions will have provided little opportunity for proper evaluation.
Championship leader Casey Stoner was fastest for a long while, before Simoncelli and Rossi moved back into first and second and edged the Australian down to third, just ahead of his Honda teammate Andrea Dovizioso.
Jorge Lorenzo was top Yamaha in fifth place, the factory bikes carrying a special red and white color scheme this weekend to celebrate the marque's 50th year in grand prix racing.
Cal Crutchlow briefly got as high as third in a remarkable performance on the Tech 3 Yamaha. The Briton's participation in this event was in doubt after surgery on the complex collarbone break he suffered in Silverstone qualifying, but he completed 13 laps this morning and was sixth at the end of the session.
Karel Abraham (Cardion AB Ducati), Colin Edwards (Tech 3), Nicky Hayden (Ducati) and Ben Spies (Yamaha) completed the top 10.
Hiroshi Aoyama was 12th in his first session on the factory Honda usually earmarked for Dani Pedrosa, while Aoyama's Gresini stand-in Kousuke Akiyoshi was 16th.
Pos Rider Team/Bike Time Gap
1. Marco Simoncelli Gresini Honda 1m49.036s
2. Valentino Rossi Ducati 1m49.396s + 0.360s
3. Casey Stoner Honda 1m49.527s + 0.491s
4. Andrea Dovizioso Honda 1m49.640s + 0.604s
5. Jorge Lorenzo Yamaha 1m49.995s + 0.959s
6. Cal Crutchlow Tech 3 Yamaha 1m50.386s + 1.350s
7. Karel Abraham Cardion Ducati 1m50.413s + 1.377s
8. Colin Edwards Tech 3 Yamaha 1m50.968s + 1.932s
9. Nicky Hayden Ducati 1m51.277s + 2.241s
10. Ben Spies Yamaha 1m51.590s + 2.554s
11. Hector Barbera Aspar Ducati 1m52.646s + 3.610s
12. Hiroshi Aoyama Honda 1m53.132s + 4.096s
13. Alvaro Bautista Suzuki 1m53.164s + 4.128s
14. Toni Elias LCR Honda 1m53.323s + 4.287s
15. Loris Capirossi Pramac Ducati 1m53.348s + 4.312s
16. Kousuke Akiyoshi Gresini Honda 1m53.353s + 4.317s
17. Randy de Puniet Pramac Ducati 1m53.562s + 4.526s