Formula One newcomer Timo Glock put Toyota on top of the testing timesheets on Wednesday after he cast doubt on the performance of their new car.
Glock, who has replaced fellow German Ralf Schumacher at the under-performing team, lapped a damp Jerez circuit with a best time of one minute 19.779 seconds.
With Red Bull launching their new Renault-powered RB4 car before the session started, four teams had their 2008 cars in action.
Britain's championship runner-up Lewis Hamilton was the second quickest, despite twice bringing out the red flags by beaching his new McLaren in the gravel, in 1:20.099.
The Briton's former team mate Fernando Alonso, the Spanish double world champion who has left McLaren to return to Renault, was third.
Glock completed 96 laps, more than anyone, and his time provided a marked contrast to Tuesday when he warned that Toyota might take a step backwards while they came to grips with a car that differs noticeably from last year's.
"I believe we have made some significant improvements to the new car," said the driver, whose team denied he was running light on fuel.
"I think we made a step forward today. Once again the car was extremely reliable and that allowed us to complete a lot of laps and gather plenty of information."
HAMILTON HAPPY
Hamilton was happy with McLaren's pace.
"I think we've got off to a really good start," he said. "We're moving in the right direction and we need to keep on chipping away."
With traction control and other so-called driver aids now banned for the season that starts in Australia on March 16, Hamilton was one of several drivers to skid off.
"It was just wet, I touched the kerb and just went on to the edge of the gravel," he said. "Without these controls helping you on the entry to the corners, there is a lot more locking of the rear wheels and when you are on the limit and pushing that is what happens."
Ferrari's world champion Kimi Raikkonen ended the afternoon only eighth fastest but the Finn sounded upbeat.
"Overall we are pretty happy with how everything has gone and it's a good feeling from the car," he told reporters. "The car already feels pretty good so once we get the final car I think it's going to be very good."
Briton Anthony Davidson failed to get any action with Super Aguri due to a series of setbacks.
The struggling team had to fly out new parts after their car overheated on Tuesday but the plane bringing them was diverted to Malaga. They were then further held up by a motorway closure, arriving after the test had finished.
