Testing began at 10am and concludes at 6pm, but rain has been falling since 12pm.
Hector Barbera was the first rider to try some wet laps, until he fell from his Pramac Ducati at 1:15pm. Yamaha's Ben Spies plus Avintia team-mates Ivan Silva and Yonny Hernandez then went out on wet weather tyres shortly after 3pm.
Repsol Honda riders Casey Stoner and Dani Pedrosa - first and second on day one - plus fellow RC213V riders Alvaro Bautista (Gresini) and Stefan Bradl (LCR) have not been on track. The official explanation is yet to be given by HRC.
Karel Abraham (Cardion AB Ducati) withdrew from the test on day one due to pain from shoulder and back injuries sustained at Jerez last week.
The second Sepang test finishes on Thursday evening, after which only one more official test (at Jerez) will remain before the start of the racing season.
for pictures from the test.
1. Ben Spies
USA Yamaha Factory Racing 2m 1.285s
2. Jorge Lorenzo
ESP Yamaha Factory Racing 2m 1.293s
3. Andrea Dovizioso
ITA Monster Yamaha Tech 3 2m 1.522s
4. Cal Crutchlow
GBR Monster Yamaha Tech 3 2m 1.695s
5. Nicky Hayden
USA Ducati Team 2m 1.748s
6. Hector Barbera
ESP Pramac Racing 2m 1.989s
7. Valentino Rossi
ITA Ducati Team 2m 2.130s
8. Franco Battaini
ITA Ducati Test Rider 2m 3.566s
9. Colin Edwards
USA Forward Racing (Suter-BMW CRT) 2m 5.816s
10. Ivan Silva
ESP Avintia (FTR-Kawasaki CRT) 2m 7.813s
11. Yonny Hernández
COL Avintia (FTR-Kawasaki CRT) 2m 8.213s
Fastest day one time:
AUS Honda 2m 1.761s
Fastest Sepang I test times (Feb 2):
AUS Honda 1m 59.607s
ESP Yamaha 2m 0.198s
ITA Ducati 2m 0.824s
USA Suter 2m 4.722s
ESP FTR 2m 8.225s
Official Sepang MotoGP lap records:
ITA Yamaha 2m 0.518s
Fastest race lap:
AUS Ducati 2m 2.108s