Honda's Marco Simoncelli was fastest in practice for the Assen MotoGP as an oil spill cancelled the afternoon session.
The Italian lapped the Dutch circuit in a time of one minute 49.036 seconds to put his Honda on top in the rain.
Nine-time world champion Valentino Rossi was second on his new improved Ducati, with championship leader Casey Stoner in third.
British rider Cal Crutchlow impressively took sixth place, a week after surgery on his broken collarbone.
Crutchlow broke his left collarbone in five places after a crash in qualifying at Silverstone a fortnight ago and has had a metal plate inserted but managed to complete 13 laps on his Tech 3 Yamaha and was over half a second quicker than his team-mate Colin Edwards, who was eighth.
"The shoulder feels quite sore and I'm pretty cautious with it," Crutchlow explained.
"I'm definitely aware of it and when I start to go fast I think I should slow down a bit. In the wet conditions it doesn't hurt that much, but I'm really wary because the last thing I need is another crash on it.
"To be honest, I hope it rains all weekend because it will make it easier on me.
"If it's a dry race then it will be a long 26 laps on Saturday with a plate and a load of screws in my collarbone, but I will do my best as always."
Italian Rossi has struggled to match his usual pace so far this season - his first on the Ducati - but is riding a new bike this weekend and was instantly faster as he traded top spot with Simoncelli in the opening session.
The field were due to take part in a second practice session on Thursday afternoon but it was cancelled after an incident in Moto2 practice.
Alex Baldolini remounted after a crash and rode round for half a lap before pulling in, depositing oil around large areas of the circuit.
Track officials spent over two hours attempting to clean the surface but decided that the combination of oil and water was too dangerous and called off the planned sessions for MotoGP, Moto2 and 125cc bikes.
Dutch MotoGP practice times:
1. M Simoncelli (Ita) Gresini Honda 1'49.036
2. V Rossi (Ita) Ducati 1'49.396
3. C Stoner (Aus) Repsol Honda 1'49.527
4. A Doviziosos (Ita) Repsol Honda 1'49.640
5. J Lorenzo (Spa) Yamaha 1'49.995
6. C Crutchlow (GB) Tech 3 Yamaha 1'50.386
7. K Abraham (Cze) Cardion Ducati 1'50.413
8. C Edwards (USA) Tech 3 Yamaha 1'50.968
9. N Hayden (USA) Ducati 1'51.277
10. B Spies (USA) Yamaha 1'51.590
11. H Barbera (Spa) Aspar Ducati 1'52.646
12. H Aoyama (Jpn) Repsol Honda 1'53.132
13. A Bautista (Spa) Suzuki 1'53.164
14. T Elias (Spa) LCR Honda 1'53.323
15. L Capirossi (Ita) Pramac Ducati 1'53.348
16. K Akiyoshi (Jpn) Gresini Honda 1'53.353
17. R De Puniet (Fra) Pramac Ducati 1'53.562