Lewis Hamilton led teammate Heikki Kovalainen in a McLaren-Mercedes one-two in a damp first practice session for the German Grand Prix at Hockenheim.
Ferrari duo Felipe Massa and Kimi Raikkonen ended up third and fifth, sandwiching Renault's Fernando Alonso, as positions changed constantly in the latter stages as the track conditions improved.
The track had remained wet for the first half of the session, with early morning rain being followed by a further shower and drizzle shortly after practice began.
Sebastian Vettel (Toro Rosso), Hamilton and Alonso - all on intermediates - swapped the top spot initially, with Hamilton escaping a quick spin across the grass at Turn 4 without drama or damage, before the Ferraris moved to the front after 35 minutes.
With the rain having eased off by that time, each new flying lap set a new benchmark, as times tumbled and positions changed relentlessly.
It was Vettel who led the charge for dry weather tyres as the session reached the halfway point. His first lap was still one second slower than Massa was managing on intermediates at the time, but within another three laps Vettel's slicks were up to temperature and he was 3.4 seconds ahead of the rest of the field.
Surprisingly, another ten minutes elapsed before most of the pack joined Vettel on dries. His Toro Rosso teammate Sebastien Bourdais was among the first to change tyres and briefly held second place, before pulling off on the far side of the circuit with a mechanical problem.
Once the whole field started using dry tyres, the order went predictably haywire, with first place changing hands 13 times in the final half hour of the session. All four German drivers managed a moment on top in quick succession, including Force India's Adrian Sutil, with both Ferraris, Mark Webber and Hamilton also taking turns to lead the way as the track became ever faster.
It looked like Massa was going to end the morning at the end of the pack, but in the closing seconds Kovalainen burst out of the midfield - having taken his time to set a flying lap on dries - and beat the Ferrari by 0.130 seconds, before being usurped by teammate Hamilton moments later.
That left Massa third ahead of Alonso and Raikkonen, with Williams' Nico Rosberg - who had been fastest on several occasions as the track dried - completing the top six.
Vettel ended the morning seventh, followed by Heidfeld, Kazuki Nakajima (Williams) and Renault's Nelson Piquet.
Although many drivers slid across the run-off areas in the treacherous conditions, Robert Kubica was the only man who made it as far as the wall. A spin entering the stadium section with 11 minutes to go saw his BMW bounce across the gravel and gently hit the barriers side-on, ending his session and leaving him at the foot of the timesheets with Bourdais.
Pos Driver Team Time Laps
1. Hamilton McLaren-Mercedes (B) 1:15.537 22
2. Kovalainen McLaren-Mercedes (B) 1:15.666 + 0.129 19
3. Massa Ferrari (B) 1:15.796 + 0.259 22
4. Alonso Renault (B) 1:16.163 + 0.626 22
5. Raikkonen Ferrari (B) 1:16.327 + 0.790 23
6. Rosberg Williams-Toyota (B) 1:16.606 + 1.069 27
7. Vettel Toro Rosso-Ferrari (B) 1:16.618 + 1.081 25
8. Heidfeld BMW Sauber (B) 1:16.719 + 1.182 20
9. Nakajima Williams-Toyota (B) 1:16.821 + 1.284 26
10. Piquet Renault (B) 1:17.063 + 1.526 26
11. Coulthard Red Bull-Renault (B) 1:17.108 + 1.571 18
12. Button Honda (B) 1:17.131 + 1.594 28
13. Glock Toyota (B) 1:17.185 + 1.648 28
14. Webber Red Bull-Renault (B) 1:17.268 + 1.731 11
15. Fisichella Force India-Ferrari (B) 1:17.471 + 1.934 30
16. Barrichello Honda (B) 1:17.500 + 1.963 24
17. Trulli Toyota (B) 1:17.556 + 2.019 29
18. Sutil Force India-Ferrari (B) 1:17.784 + 2.247 29
19. Kubica BMW Sauber (B) 1:18.779 + 3.242 8
20. Bourdais Toro Rosso-Ferrari (B) 1:21.506 + 5.969 16
All Timing Unofficial