Finland's Heikki Kovalainen had his first proper taste of McLaren's new MP4-24 formula one car at the Jerez circuit in southern Spain in a test which also highlighted Red Bull's promise.
The Finn, who was unable to complete a lap in the car last month due to bad weather in Portugal, was fourth fastest of the five drivers in action.
Red Bull's Sebastian Vettel, whose new RB5 car was unveiled at the circuit on Monday, was quickest of those in the 2009 machinery with 63 laps.
German test driver Nico Hulkenberg was next for Williams, according to unofficial times, and Kovalainen completed 60 laps.
McLaren say they had run with the new KERS energy recovery device and had set prudent targets.
Red Bull's Australian Mark Webber is due to return to the cockpit tomorrow for the first time since he broke his leg in November.
Champions Ferrari, BMW-Sauber and Toyota are testing in Bahrain where the start of the session was delayed for nearly two hours by fog.
Toyota's Timo Glock was fastest of the three drivers at the Sakhir circuit.