McLaren's Jenson Button was fastest from Lotus's Romain Grosjean in practice at the Monaco Grand Prix.
The Englishman was one of very few drivers to run the faster of the two tyre types, before a rain shower 15 minutes into the afternoon session.
Ferrari's Fernando Alonso earlier set the pace in the first session, run in beautiful morning sunshine, by 0.365 secs from Grosjean.
Intermittent spells of rain wrecked the rest of the second session.
That meant Button's fastest time of one minute 15.746 seconds, which was 0.392 secs quicker than Grosjean, held for the rest of the afternoon.
Button, whose team-mate Lewis Hamilton was 11th fastest in the afternoon, said: "It's not a lot of running for a Thursday at Monaco. Hopefully Saturday will be a bit better."
The forecast for qualifying day on Saturday is that there is an 80% chance of rain, however.
Ferrari's Felipe Massa, showing improved form after being warned by his team to up his game following disappointing performances so far this season, was third fastest in the second session, just ahead of team-mate Alonso.
Williams driver Pastor Maldonado, who also ran the faster 'super-soft' tyre, was fifth fastest, ahead of Nico Rosberg's Mercedes and Red Bull's Mark Webber.
Sauber's Kamui Kobayashi was eighth fastest, ahead of Rosberg's team-mate Michael Schumacher and world champion Sebastian Vettel's Red Bull.
Button was 11th fastest in the morning session, which Alonso topped by 0.365 secs from Grosjean. Sauber's Sergio Perez was 0.081 secs behind the Frenchman and Hamilton was 0.036 secs further back in fourth.
Alonso said afterwards
"Good Thursday in Monaco, gaining confidence with the car. Happy."
The first session came to a premature end when the engine in Heikki Kovalainen's Caterham blew up in the tunnel with seven minutes remaining.
Maldonado, who won the last race in Spain, was fifth, ahead of Massa, Kobayashi, Button, Vettel and Rosberg.
The mixed line-ups in both sessions bode well for another competitive, unpredictable race.
"This year's racing is really competitive," said BBC F1 technical analyst Gary Anderson. "That's seven different cars in the top 10 and any one of those guys could win the race.
"It bodes so well for the different chassis having some success. We've got two Ferraris, Saubers and McLarens up there. The first Red Bull comes in ninth. Even Lotus and Williams are looking strong."
Alonso, Grosjean and Hamilton, in particular, looked impressive out on the track in the dry.
They swapped fastest times throughout the first session, with the Lotus going fastest, then the Ferrari before Hamilton took top spot.
Grosjean then took it back, before Alonso finally grabbed it for good with a lap in one minute 16.265 seconds.
Two-time world champion Mika Hakkinen, watching at the second Swimming Pool chicane, told Anderson: "Monaco is a difficult circuit to drive because the barriers are so close.
"You need to have the perfect run and have maximum concentration to get a good lap. It looks like the Lotus is really strong here. Nico Rosberg is looking confident as well but it's still too early to say."
Grosjean said: "I like Monaco. I like the track; it's good fun. The car is going well at the moment and let's see what we can do later on. It is important to have a car you are confident with."
Grosjean's team-mate Kimi Raikkonen failed to set a lap time after asking for a change of steering rack because he did not like the feel of the first one.
The Finn, who has not raced in Monaco since 2009, was 19th fastest in the second session.
Raikkonen looked impressive when he ran on intermediate tyres on a damp track at the end - two seconds faster than other drivers who were out there running at the same time.
Monaco Grand Prix 2012, day one
- Thursday 24 May: First practice 0855-1035 BST, Second practice 1255-1435. BBC Red Button, BBC Radio 5 live sports extra and live text commentary on BBC Sport website.
Monaco Grand Prix 2012, day two
- Saturday 26 May: Third practice 0955-1105 BST BBC Red Button, BBC Radio 5 live sports extra and live text commentary on BBC Sport website; Qualifying 1300 BST BBC Radio 5 live, 1210 BST BBC One and BBC One HD and live text commentary on BBC Sport website.
Monaco Grand Prix 2012, day three
- Sunday 27 May: Race 1205 BST BBC One and BBC One HD, 1300 BBC Radio 5 live and live text commentary on BBC Sport website; Highlights 1900-1950 BST BBC Three.