Jenson Button set the fastest time of the opening practice session at Monza, though there was little to choose between the pace of the McLaren and Red Bull.
The reigning World Champion's best time was under a tenth quicker than second-placed Sebastian Vettel, with Lewis Hamilton the only other driver in the 1m23sec in third.
The 90-minute session was half over before anyone set a competitive time, as teams used the early running to compare different setups. Toro Rosso tried its F-duct on a grand prix weekend for the first time, meaning that Lotus, Virgin and Hispania are now the only teams without the innovation.
McLaren began the session with a larger rear wing, similar to its Spa setup, while Red Bull used its first runs to test the RB6's low downforce aero efficiency with high-visibility paint.
Hamilton was the first driver to set a representative time, though the target time was lowered several times before Button hit the top. Robert Kubica and Nico Rosberg were fourth and fifth quickest for Renault and Mercedes, while Mark Webber's Red Bull was sixth.
Vitantonio Liuzzi was next up for Force India, ahead of the Ferraris of Fernando Alonso and Felipe Massa, with Michael Schumacher completing the top 10. Nico Hulkenberg was 11th for Williams, while Paul di Resta showed well in 12th on his latest practice outing for Force India. Vitaly Petrov's Renault then headed the two Saubers and Toro Rossos.
Timo Glock was fastest of the new teams; his Virgin eight tenths off the back of Jaime Alguersuari and a tenth ahead of Jarno Trulli's Lotus and teammate Lucas di Grassi. Heikki Kovalainen lost a significant chunk of running when the hydraulics on his Lotus failed on his installation lap. He ended up 21st.
Bruno Senna's participation was also delayed after the Hispania team discovered a brake fluid leak on his car. He joined the session later but stopped out on track after just eight laps.
Rubens Barrichello was the session's other casualty when he parked up exiting the second Lesmo halfway through with a loss of drive.
Pos Driver Team Time Laps
1. Button McLaren-Mercedes 1:23.693 28
2. Vettel Red Bull-Renault 1:23.790 + 0.097 27
3. Hamilton McLaren-Mercedes 1:23.967 + 0.274 25
4. Kubica Renault 1:24.120 + 0.427 25
5. Rosberg Mercedes 1:24.129 + 0.436 30
6. Webber Red Bull-Renault 1:24.446 + 0.753 26
7. Liuzzi Force India-Mercedes 1:24.512 + 0.819 19
8. Alonso Ferrari 1:24.543 + 0.850 24
9. Massa Ferrari 1:24.648 + 0.955 22
10. Schumacher Mercedes 1:24.756 + 1.063 26
11. Hulkenberg Williams-Cosworth 1:24.841 + 1.148 28
12. di Resta Force India-Mercedes 1:24.923 + 1.230 23
13. Petrov Renault 1:25.292 + 1.599 25
14. Buemi Toro Rosso-Ferrari 1:25.318 + 1.625 29
15. de la Rosa Sauber-Ferrari 1:25.320 + 1.627 20
16. Kobayashi Sauber-Ferrari 1:25.334 + 1.641 24
17. Alguersuari Toro Rosso-Ferrari 1:25.897 + 2.204 19
18. Glock Virgin-Cosworth 1:26.772 + 3.079 19
19. Trulli Lotus-Cosworth 1:26.898 + 3.205 12
20. di Grassi Virgin-Cosworth 1:26.956 + 3.263 17
21. Kovalainen Lotus-Cosworth 1:27.374 + 3.681 14
22. Senna HRT-Cosworth 1:28.256 + 4.563 8
23. Barrichello Williams-Cosworth 1:28.516 + 4.823 4
24. Yamamoto HRT-Cosworth 1:29.870 + 6.177 17
All Timing Unofficial