Nakjima was joined on track by 2008 team mate Nico Rosberg, fifth, and Williams tester Nico Hulkenberg, 13th. While Rosberg enjoyed sole use of one of the team's FW30s, completing 109 laps, Nakajima and Hulkenberg shared the second to cover 32 and 59 laps respectively.
Barely three-hundredths of a second down on Nakajima was Heikki Kovalainen for McLaren. Having shied away from the damp conditions earlier in the week, it was a successful first day for the British team, with team tester Pedro de la Rosa finishing in third.
Wednesdays quickest runner, Felipe Massa, took up fourth slot for Ferrari, while the Italian teams tester Luca Badoer finished eighth in the second F2008. Red Bulls Mark Webber and David Coulthard, meanwhile, ended the day in sixth and seventh respectively as they continued to prepare the RB4 for the seasons opening race in March.
Also enjoying top-ten placings were Fernando Alonso in ninth for Renault and Timo Glock in 10th for Toyota. While Alonso was joined by fellow race driver Nelson Piquet Jr, 11th, Glock went out on track alongside team tester Kamui Kobayashi, 12th.
Glock covered 109 laps during the session as he worked through a programme focusing on the TF108s new aerodynamic package. Kobayashi carried out engine and set-up work and completed 56 laps.
“Finally we had a day of running in the dry today so we could at last begin meaningful work on our new aerodynamics package,” exclaimed Dieter Gass, Toyota's chief race and test engineer. “It was still a difficult day because there were many disruptions for red flags.
"But on Timos car we made it through the programme of work that we had pencilled in for the first day of this weeks test. Kamui also made it through his programme and collected some useful data.”
Most teams will now leave the Circuit de Catalunya for a brief period before returning for a second three-day test next week from February 25. Ferrari, who only joined proceedings on Wednesday, are the exception and are expected to continue testing on Friday.
Unofficial Thursday times from Barcelona:
1. Kazuki Nakajima, Williams, 1:22.153
2. Heikki Kovalainen, McLaren, 1:22.185
3. Pedro de la Rosa, McLaren, 1:22.208
4. Felipe Massa, Ferrari, 1:22.213
5. Nico Rosberg, Williams, 1:22.248
6. Mark Webber, Red Bull, 1:22.477
7. David Coulthard, Red Bull, 1:22.499
8. Luca Badoer, Ferrari, 1:22.535
9. Fernando Alonso, Renault, 1:22.657
10. Timo Glock, Toyota, 1:22.901
11. Nelson Piquet Jr, Renault, 1:23.286
12. Kamui Kobayashi, Toyota, 1:24.132
13. Nico Hulkenberg, Williams, 1:24.222
