Fernando Alonso led the way on a rain-affected first day of Formula One testing at the Mugello circuit in Italy.
The Ferrari driver's best time of one minute 22.444 seconds, set this morning on a drying track following overnight rain and fog, was enough to put him comfortably clear of the rest of the pack before heavier showers set in again this afternoon.
The dismal weather prompted circuit officials to red flag the session on safety grounds this afternoon, and many teams elected to remain in their garages even after the track was re-opened.
The conditions curtailed a first chance in four years for teams to take part in official mid-season testing, although better weather is forecast tomorrow and Thursday.
With 11 of the 12 F1 teams in attendance at the three-day test, the coming days should be highly competitive, with little to choose between many of the front-running outfits at the opening races of the season in Australia, Malaysia, China and Bahrain.
Alonso was one of four different race winners at the opening four rounds, as was Mercedes' Nico Rosberg, who managed the highest number of laps of any driver today - 49 - but could do no better than fifth place, 1.6secs slower than the Ferrari man.
"It was a good session for me this morning and a positive start to our testing programme," said China race-winner Rosberg.
"It's nice to see that we have some new parts for the car and that the ongoing process of in-season developments is working well."
Red Bull's Mark Webber took second with a time of 1min 23.648secs, while Toro Rosso's Jean-Eric Vergne was third in 1:23.891.
Lotus test driver Jerome d'Ambrosio was fourth, while Sauber's Kamui Kobayashi was sixth ahead of a string of test drivers in Oliver Turvey (McLaren), Jules Bianchi (Force India) and Rodolfo Gonzalez (Caterham).
Marussia's Charles Pic was 10th ahead of Valtteri Bottas (Williams) and Gary Paffett (McLaren), while the wet conditions meant Michael Schumacher was unable to set a timed lap for Mercedes after taking over from Rosberg this afternoon.