Front row qualifier Cal Crutchlow finished fastest during a wet warm-up session for the Catalunya MotoGP round on Sunday morning.
After four roasting hot track sessions, light rain began to fall just before the start of the 20-minute warm-up.
Some riders chose to try some exploratory laps on slicks at the start of the session, but returned straight back to the pits and the only complete laps recorded were with wet tyres.
The session was briefly red-flagged with five minutes remaining to allow marshals to retrieve James Ellison's crashed PBM Aprilia from the Turn Five gravel trap.
After the restart Dani Pedrosa lowsided from his Repsol Honda under braking at Turn 10. Jorge Lorenzo, Valentino Rossi, Hector Barbera and Karel Abraham ran off at the same turn, but stayed upright.
Top CRT qualifier Randy de Puniet (Aspar) joined Ellison in crashing out.
Tech 3 Yamaha rider Crutchlow finished the session 0.044s ahead of reigning world champion Casey Stoner (Honda) with Ducati's Valentino Rossi 0.497s from the top in third.
Tyre choice was already borderline between the hard or soft rear slick compound in dry conditions, and the change of weather throws another unknown into the equation.
However it is far from guaranteed that the wet weather will continue and the track could yet dry in time for this afternoon's race.
Home star Lorenzo will start round five from second on the grid and with an eight point lead over pole sitter Stoner.
Sunday's rain means that all four rounds since Qatar have experienced at least one wet session.
1. Cal Crutchlow
2. Casey Stoner
3. Valentino Rossi
4. Alvaro Bautista
5. Jorge Lorenzo
6. Stefan Bradl
7. Dani Pedrosa
8. Yonny Hernandez
9. Danilo Petrucci
10. Ivan Silva
11. Michele Pirro
12. Ben Spies
13. Andrea Dovizioso
14. Nicky Hayden
15. Hector Barbera
16. Mattia Pasini
17. Aleix Espargaro
18. Randy De Puniet
19. Colin Edwards
20. Karel Abraham
21. James Ellison