Pramac Ducati's Andrea Iannone topped the second Brno MotoGP practice session by virtue of getting a flying lap in during the tiny period of almost-dry running.
Iannone and Avintia rider Hector Barbera were the only two riders who set times before the drizzle that began with the green light became proper rainfall.
Although the shower did not last, the track never dried out enough for Iannone and Barbera's slick-shod times to be beaten.
Marc Marquez got closest, but his 2m07.846s in third place was 4.667 seconds behind Iannone.
Championship leader Marquez edged out Honda team-mate Dani Pedrosa by just 0.002s to take 'wet track' honours, with the pair a second quicker than Valentino Rossi's Yamaha in fifth.
Cal Crutchlow went well in the wet, putting his Ducati sixth ahead of Jorge Lorenzo.
Barbera had the session's only accident, crashing in the final minute, while Iannone had his own trip through the gravel but stayed upright.
With rain a possibility throughout the weekend, everyone went out for a decent amount of wet running rather than hiding in the pits, the whole field completing at least 10 laps each.
Pos Rider Bike Time Gap Laps 1. Andrea Iannone Pramac Ducati 2m03.179s 12 2. Hector Barbera Avintia FTR-Kawasaki 2m04.462s +1.283s 12 3. Marc Marquez Honda 2m07.846s +4.667s 12 4. Dani Pedrosa Honda 2m07.848s +4.669s 13 5. Valentino Rossi Yamaha 2m08.827s +5.648s 12 6. Cal Crutchlow Ducati 2m08.981s +5.802s 11 7. Jorge Lorenzo Yamaha 2m09.114s +5.935s 14 8. Pol Espargaro Tech 3 Yamaha 2m09.412s +6.233s 15 9. Bradley Smith Tech 3 Yamaha 2m09.722s +6.543s 15 10. Stefan Bradl LCR Honda 2m09.747s +6.568s 13 11. Scott Redding Gresini Honda 2m09.869s +6.690s 13 12. Andrea Dovizioso Ducati 2m10.174s +6.995s 10 13. Yonny Hernandez Pramac Ducati 2m10.217s +7.038s 13 14. Danilo Petrucci Ioda Aprilia 2m10.264s +7.085s 12 15. Aleix Espargaro Forward Yamaha 2m10.556s +7.377s 14 16. Karel Abraham Cardion Honda 2m10.668s +7.489s 12 17. Michele Pirro Ducati 2m10.828s +7.649s 11 18. Alvaro Bautista Gresini Honda 2m11.004s +7.825s 14 19. Hiroshi Aoyama Aspar Honda 2m11.019s +7.840s 15 20. Michael Laverty PBM-Aprilia 2m11.337s +8.158s 10 21. Broc Parkes PBM-Aprilia 2m11.741s +8.562s 10 22. Mike di Meglio Avintia FTR-Kawasaki 2m12.270s +9.091s 12 23. Leon Camier Aspar Honda 2m12.934s +9.755s 14 24. Alex de Angelis Forward Yamaha 2m15.514s +12.335s 13