Sebastian Vettel edged out Fernando Alonso in a late battle for the honour of being fastest in the opening practice session for the Monaco Grand Prix.
Both set their fastest laps moments apart in the closing seconds, with Vettel's 1m16.619s for Red Bull sufficient to nip ahead of his Ferrari rival by 0.113 seconds.
Nico Rosberg was third for Mercedes, followed by Alonso's team-mate Felipe Massa and the two McLarens.
But last year's winner Mark Webber did not have a good start to his weekend. A gearbox issue on his Red Bull meant he only completed three slow installation laps and did not set a time.
Unsurprisingly as circuit conditions got ever better, a string of different drivers topped the times during the session, with both Ferraris, Hamilton and Vettel leading the way before Rosberg put Mercedes on top going into the final five minutes. He could not stay there once Alonso and Vettel launched their last runs though, and ended up half a second down in third.
Behind Massa, Lewis Hamilton was just ahead of McLaren team-mate Jenson Button in fifth.
Pastor Maldonado - who has an outstanding record in Monaco from his junior career - showed similar form on his first taste of the circuit in a Formula 1 car, taking a very promising seventh for Williams.
Two drivers found the barriers during the session, the first being Tonio Liuzzi, who crashed his Hispania on the approach to the chicane.
The other accident was courtesy of Michael Schumacher. He locked up on the approach to Sainte Devote, appeared to make a late decision to try and take to the escape road, but ended up spinning sideways into the barriers. Hispania's Narain Karthikeyan went off in sympathy as he watched the incident, though the Indian avoided contact.
Both of those incidents were swiftly cleared away with no need for a stoppage, but there was a five-minute red flag mid-session while the officials investigated a puddle of water that had appeared under the flagstand on the main straight, seemingly from an under-surface leak.
Pos Driver Team Time Laps 1. Sebastian Vettel Red Bull-Renault 1m16.619s 25 2. Fernando Alonso Ferrari 1m16.732s + 0.113 24 3. Nico Rosberg Mercedes 1m17.139s + 0.520 20 4. Felipe Massa Ferrari 1m17.316s + 0.697 24 5. Lewis Hamilton McLaren-Mercedes 1m17.350s + 0.731 23 6. Jenson Button McLaren-Mercedes 1m17.534s + 0.915 24 7. Pastor Maldonado Williams-Cosworth 1m18.527s + 1.908 30 8. Adrian Sutil Force India-Mercedes 1m18.578s + 1.959 24 9. Vitaly Petrov Renault 1m18.733s + 2.114 16 10. Michael Schumacher Mercedes 1m18.805s + 2.186 14 11. Nick Heidfeld Renault 1m18.928s + 2.309 19 12. Sebastien Buemi Toro Rosso-Ferrari 1m19.234s + 2.615 24 13. Rubens Barrichello Williams-Cosworth 1m19.395s + 2.776 24 14. Daniel Ricciardo Toro Rosso-Ferrari 1m19.463s + 2.844 25 15. Kamui Kobayashi Sauber-Ferrari 1m19.768s + 3.149 25 16. Sergio Perez Sauber-Ferrari 1m19.792s + 3.173 26 17. Heikki Kovalainen Lotus-Renault 1m20.083s + 3.464 23 18. Jarno Trulli Lotus-Renault 1m21.116s + 4.497 27 19. Paul di Resta Force India-Mercedes 1m21.548s + 4.929 32 20. Jerome D'Ambrosio Virgin-Cosworth 1m21.758s + 5.139 31 21. Timo Glock Virgin-Cosworth 1m21.815s + 5.196 17 22. Tonio Liuzzi HRT-Cosworth 1m22.840s + 6.221 13 23. Narain Karthikeyan HRT-Cosworth 1m23.885s + 7.266 37 24. Mark Webber Red Bull-Renault No time 3 All Timing Unofficial