Nico Rosberg was happy with Mercedes' new 2011 car on the first day of testing at Valencia.
The German and his famous teammate Michael Schumacher put the first miles on the W02 at the Spanish track but were not close to the top of the times.
Rosberg also had a technical problem and stoppage on the front straight, but declared himself happy anyway.
"I felt at home straight away," he said, "and that's important."
Tuesday's fastest runner was world champion Sebastian Vettel in the new Red Bull, nearly a second faster than the next best runner in a 2011 car, Ferrari's Fernando Alonso.