BMW's Nick Heidfeld topped the timesheets on the first day of testing at Barcelona, but it was Jenson Button and Brawn GP who impressed, dominating proceedings for much of the day.

After early showers the track started off partly wet but there was no further rain and in spite of cool temperatures the tarmac soon dried out.

Heidfeld concentrated on race related procedures such as pit stop practice, as BMW seeks to fine tune the working relationship between the driver, engineers and mechanics. The German, who continues here tomorrow before handing over to Robert Kubica for the final two days of the test, also did a number of practice starts.

"The weather was perfect for today's programme," said Heidfeld after completing 92 laps. "We could try a lot of different set-up solutions and even do some aero work."

Second quickest was Kimi Raikkonen as he worked on finding the best set-up for the F60, on reliability and on a comparison between various aerodynamic solutions. In the morning, the Finn completed Just 25 laps before the lunch break, due to several stops while the team carried out checks on the car's underbody.

The Maranello outfit claims today's times weren't very "expressive", because the F60 was out for just one lap on the regular tyres.

Jarno Trulli was at the helm of the TF109, working on a programme that included aerodynamics, brakes and set-up. He had a successful day, putting in over 120 laps.

"This morning started off with drizzly conditions," said the Italian, "but the rain quickly tailed off, giving us the chance to have a good day's work. The afternoon was windier but the track surface was fine. The car felt well balanced from the start and we worked on several new things on the aero side which gave us positive results. We were competitive throughout the day and we collected a lot of data so this was a useful start to the test."

"Today we carried out a lot of different tests," added Dieter Gass, the team's Chief Engineer Race and Test, "mainly on new aerodynamic components but also on brakes and suspension. So it was a full programme and there were no technical problems. Conditions were good and we made it through everything we had planned. We did a lot of laps so it was a positive start to our last week of preparation for Melbourne."

Brawn GP's car, the BGP 001, made its full track debut following an initial shakedown at Silverstone last Friday.

The car showed encouraging reliability and pace as the team intensifies its preparations for the season-opening Australian Grand Prix in just over two weeks. With only seven days of testing between now and Melbourne, the Brackley team's test programme has been carefully planned to ensure a balance between reliability checks and improving the overall performance of the car.

Driving responsibilities over the four-days will be shared by Jenson Button and Rubens Barrichello. After shaking down the car at Silverstone, Button had the opportunity to evaluate the car on a full Grand Prix circuit for the first time. In addition to completing a reliability and set-up programme with the new chassis, he worked through new tyre evaluations using the new Potenza slicks and completed a race start simulation.

The Englishman completed 82 laps over the course of the day and set the fourth fastest time overall. Team-mate Rubens will join the Brawn GP team in Barcelona tomorrow to take over the BGP 001.