Force India's Paul Di Resta in action at Jerez
Young Scot Paul Di Resta made a solid start as Force India demonstrated it is in competitive shape during a rain-affected day's testing in Spain.
A stop-start morning's running saw red flags waved twice in the first hour.
Lewis Hamilton's McLaren stopped on the track and Di Resta spun as the teams adjusted to the conditions.
Rubens Barrichello's Williams also stopped on the circuit, as did Felipe Massa's Ferrari. Last week's Jerez test was also hit by the weather.
In between the showers, the 11 teams participating in the test did their best to get the laps in.
In such variable conditions conclusions were even harder to draw than usual, but there was no doubt it was a tough day for new teams Lotus and Virgin.
Of the two, Lotus had more cause to be satisfied as test driver Fairuz Fauzy, driving without power steering according to team technical director Mike Gascoyne, logged over 70 laps, though the Malaysian's fastest lap was over nine seconds off the pace.
Lotus is back in F1
"It's been an excellent day for the whole team," said Gascoyne. "Firstly I want to say a big thank you to Fairuz who did an excellent job today with the first proper day of running. With so much going on, such as doing systems checks, it can be a pretty disjointed day for the driver.
"When you add into that the changeable conditions, and also the fact we were running without power steering today, it could have been a tricky day for Fairuz but he didn't put a foot wrong so well done to him and a big thank you.
"With the car we had no major reliability issues - a few niggles with overheating which were quickly solved and then the car ran pretty faultlessly. The power steering issue will be fixed for tomorrow so all in all a very successful first day and a busy evening ahead preparing the car for tomorrow."
The day was rather less rewarding for Virgin, with Timo Glock completing a mere five laps.
During last week's test at the same venue, Glock had completed only 11 laps when the front wing fell off his car, and Virgin had to call off any further testing due to a shortage of spare parts.
Di Resta, by contrast, ensured Force India had plenty of data to work with, clocking up 74 laps and steadily improving his times before handing over to race driver Adrian Sutil.
The 23-year-old Di Resta, a cousin of reigning Indy car champion Dario Franchitti, was confirmed as Force India's test and reserve driver after impressing in the young driver test in Jerez in December.
He will also continue to race in the German Touring Car championship (DTM) this year but will attend all 19 grands prix this season and will be allowed to run in Friday practice sessions.
With Force India race drivers Sutil and Vitantonio Liuzzi under pressure to produce results, Di Resta knows his chances of making his F1 grand prix debut this season are good.
"The objective was to get as much running as possible and we managed over 300km, which was a great effort, particularly given the weather conditions," he said.
"It seemed to be drying out and then the rain would return again so you were in a sort of no man's land with the tyres.
"Even with this type of weather though, I could get an impression of the car. It seems stable and good all round, a genuine improvement on the car I drove last year.
"I'll be back in the car in Barcelona [next week] and I am looking forward to it already."
Up at the sharp end, Sebastian Vettel (Red Bull), Felipe Massa (Ferrari), Hamilton (McLaren) and Michael Schumacher (Mercedes) were joined by Sebastien Buemi of Toro Rosso and Sauber's Pedro De La Rosa as the lap times changed with the conditions.
As the afternoon progressed, Vettel confirmed Red Bull is beginning to hit its stride, knocking Massa off the top of the timing charts by half a second.
A fifth red flag shortly before the close (the second caused by Barrichello) ensured a final rush of cars on to the circuit. Hamilton was the big improver, closing to within half a second of Vettel's best.
Schumacher, who completed an impressive 111 laps, was satisfied with the reliability of the new Mercedes.
"The weather was really changing all the time today which made it difficult to follow our planned programme and to have a clear picture of everyone's performance," the German said.
"From the laps that we achieved, we have again confirmed the reliability of the car, and from the impressions and data gathered, we can say that the pace and performance looks very good.
"We have a strong basis to keep up the development this week here in Jerez and next week at the final test in Barcelona."
Jerez testing times, day one:
1. Vettel (Red Bull) 1.22.593 ( 99 laps completed)
2. Hamilton (McLaren) 1.23.017 (72)
3. Massa (Ferrari) 1.23.204 (72)
4. Buemi (Toro Rosso) 1.23.322 (79)
5. De La Rosa (Sauber) 1.23.367 (76)
6. Schumacher (Mercedes) 1.23.803 (111)
7. Sutil (Force India) 1.24.272 (28)
8. Di Resta (Force India) 1.25.088 (74)
9. Petrov (Renault) 1.26.237 (55)
10. Barrichello (Williams) 1.27.320 (109)
11. Fauzy (Lotus) 1.31.848 (76)
12. Glock (Virgin) 1.34. 453 (10)