Valentino Rossi made the first appearance of his Yamaha return on a sodden Valencia circuit on Tuesday.
The Italian spent seven years with the Japanese squad before making his ill-fated switch to Ducati for 2010.
During Rossi's first Yamaha sojourn, he won four MotoGP titles and claimed 46 wins for the manufacturer.
Speaking before the weekend, Rossi had said he was excited to understand his potential after the first two winless seasons of his career on board the Desmosedici, and to see how he compared to champion team-mate Jorge Lorenzo.
"We are here to try and get the best results, and after two difficult seasons now we move forward," he said.
"It will be very important if we are able to do one or two dry days in [the] test to understand our potential, but I am happy [to join Yamaha].
"I'm very curious, but most of all I hope to become a problem for Lorenzo."
Click here for Toby Moody's verdict on Valentino Rossi's return to Yamaha
Bad weather has however restricted his first day of running, Rossi completing just 10 laps in the opening three hours.
That was still 10 more than Moto2 champion and new Honda works rider Marc Marquez.
Two days after his extraordinary win from the back of the field in his Moto2 farewell, the Spaniard has thus far opted to completely sit out the opening of the first post-season test.
Fellow Moto2 graduate Bradley Smith was in action, as was Andrea Dovizioso, the man he replaced at the Tech 3 Yamaha squad.
The Italian completed his first laps for Ducati, having agreed to join the works team in August.
Andrea Iannone, newly installed in Ducati's restructured satellite Pramac team, also opted to sit out the first three hours due to the rain and cold temperatures.