Red Bull's Mark Webber set the pace as the British Grand Prix got off to a damp start in Friday practice at Silverstone.
The Australian, last year's winner, was fastest in the morning session, with Ferrari's Felipe Massa on top in the afternoon, 3.3 seconds slower.
The conditions meant teams' attempts to discover how their cars responded to the latest rule change were limited.
A restriction has been introduced on the use of exhausts to boost downforce.
The use of off-throttle blowing of diffusers has been banned for the first time this weekend at Silverstone.
This is a technology by which exhaust gases are blown over the rear floor of the car even when the driver is not pressing the accelerator, to increase downforce.
And with Red Bull dominating both championships, their rivals McLaren, Ferrari and Mercedes are desperate to find out if the new rule will help them close the gap.
Ferrari's Fernando Alonso said before practice: "Everyone will suffer but we have no idea which team will be affected more or less."
Massa said he and team-mate Alonso, who was nearly three seconds adrift of Massa's top time in 15th, were running similar programmes in the afternoon although Alonso had a slight difference with the floor of his car.
On his fastest lap, Massa said: "It was the last lap of the day so the track was getting better.
"It was a difficult day because it was raining all the time. I just used on set of intermediate tyres all day because if it's raining tomorrow we will need them."
Alonso added: "We know that we have lost some downforce under braking [because of the rule changes] but it was not a problem today as we were not braking at the limit in dry conditions.
"It is very, very difficult to see any difference in the car compared to Valencia."
The mid-season rule change on exhausts has forced the teams to evaluate new ways to regain the downforce.
Red Bull daubed bright green 'flo-viz' paint over the rear of their car to assess the air flow in the morning session.
McLaren ran a new rear wing and even sent Lewis Hamilton out on a set of dry-weather tyres during the morning session, when the rest of the field were out kicking up the spray on intermediate tyres, to gather data for a "dry" set-up.
But the exact nature of the ruling was still being discussed as the cars went round the track, with Red Bull's engine supplier Renault demanding changes to the initial ruling of governing body the FIA.
A heavy rain shower heralded the start of the second session, and there was very limited action until the last 20 minutes of the 90-minute period.
Williams driver Rubens Barrichello described conditions as "difficult", saying the circuit had lots of puddles and complaining of locking brakes.
The teams tend not to run in heavy rain in practice because they are unable to use the time to discover useful information about the new parts on their cars.
The number of tyres each driver is allowed to use is also limited.
BBC 5 live analyst Anthony Davidson explained: "It's not that it's too risky, it's that there's not that much to learn.
"They all ran in the first session. They're trying to get to grips with new packages and there's no point doing aerodynamic work on a wet track.
"If the track's in this condition they have learnt all they needed to in the first session. Plus they are limited on tyres and they don't want to go out and burn tyres up."
Several drivers survived brief off-course moments, the most spectacular when Mercedes' Nico Rosberg managed to catch his car as it went into a series of slides at the first corner.
Despite the difficult conditions, the only driver to crash was Sauber's Kamui Kobayashi, who lost control at the final corner in the morning session.
Jenson Button was the top British driver in the morning session, in sixth place, with McLaren team-mate Hamilton fourth in the afternoon, one place ahead of Button.
"It was a bit of a frustrating session," said Button. "It dried out after first practice and we thought it would be nice and dry for P2 [second practice] to get some nice comparisons but no.
"We did some running in the wet but it was surprisingly slippery around here. We used the intermediate tyre not the wets.
"It was very, very slippery and maybe the reason for that is the rule change but we'll have to wait and see tomorrow. This afternoon we struggled with the balance.
"We haven't really tested [the new rear wing] so we don't know if it's giving us what we had with our previous wing. It is a different downforce level but I'm not sure we'll be running it tomorrow or not."
Hamilton, who finished fourth last time out in Valencia, was summoned to the stewards' office at the end of second practice with McLaren later explaining that they wanted clarification on why Hamilton had his DRS open in wet conditions during first practice.
The drivers are not allowed to activate the moveable rear wing during wet conditions in case it destabilises the car but Hamilton had his wing open when he was out on a risky run on soft, dry-weather tyres during the morning.
The governing body are expected to send a note ahead of the next race in Germany to clarify that drivers cannot use DRS in damp conditions even if they are on slick tyres.
Red Bull's Sebastian Vettel, who leads the championship by 77 points, was 13th and 18th in the two sessions, but the wet weather means little can be read into the times.
The British fans shrugged off a grey and rainy start to the ninth round of the season in Northamptonshire and turned out to fill the grandstands and stand trackside.
Paul di Resta took part in his first full day around Silverstone as a fully-fledged F1 driver for the Force India team, but the Scot said the wind and rain had dampened the day.
"It's the fans you feel sorry for having to put up with this," said Di Resta, who set the seventh fastest time in the second session.
It was an encouraging day for Australian Daniel Ricciardo, who spent his first session as a Hispania race driver after being loaned out by Toro Rosso for the rest of the season.
The Australian finished just 0.034secs shy of team-mate Tonio Liuzzi's best time in his first run in the car and was quicker than the Italian by 0.209secs in the afternoon.