The rain came early in Free Practice Two at Circuit de Monaco but Jenson Button struck first.
Experimenting with Pirelli's super-soft tires, the McLaren driver moved to the top of the time sheet with a quick lap of 1:15:746 just before the rain began to fall.
The wet weather began 14 minutes into the 90-minute session and continued intermittently throughout.
With the rest of P2 held on a damp surface, no driver was able to touch the 2009 World Champion's time, as Romain Grosjean clocked in second at 1:16:138, followed by Felipe Massa at 1:16:602.
P1 leader Fernando Alonso ran fourth in his Ferrari, with Formula One's newest winner, Pastor Maldonado, rounding out the top five.
Positions six through 10 on speed chart belonged to Nico Rosberg, Mark Webber, Kamui Kobayashi, Michael Schumacher and Sebastian Vettel.
Navigating a slippery track, Williams teammates Maldonado and Bruno Senna both made detours off course and down the escape road at Mirabeau. Both continued with no real damage.
Vettel, the 2011 winner of the Monaco GP and reigning World Champion, is tied with Alonso for the World Championship lead after five of 20 races.
Lewis Hamilton is third, eight points back, followed by Kimi Raikkonen (-12) and Webber (-13).
Each event of 2012 has been won by a different driver, with Button (Australia), Alonso (Malaysia), Rosberg (China), Vettel (Bahrain) and Maldonado (Spain) all scoring, respectively.
Free Practice Two concludes Thursday's on-track proceedings in Monaco.
Per Monaco tradition, there is no on-track activity scheduled for Friday in the Principality.
The action resumes with Free Practice Three at 5 a.m. ET Saturday.
SPEED.com will stream the session live.
Jared Turneris an Associate Editor for SPEED.com, covering NASCAR and Formula One, and is an Editor for TruckSeries.com. His professional motorsports writing career began in 2005.