Jenson Button topped the timesheets on the second of pre-season testing at Jerez, while world champion Sebastian Vettel's day ended prematurely.
Despite missing out on any action the previous day, Button came out firing in the official wet-weather running on Wednesday morning, completing 43 laps in his MP4-29 and setting the fastest lap time in the early stages.
The track was allowed to dry in the afternoon as the drivers switched tyres accordingly, allowing Button's former team-mate Sergio Perez in his Force India to come to the fore - though the Mexican would break down on Turn 1 after completing a number of quick laps to bring out the red flag.
Earlier, Sauber's Esteban Gutierrez also caused the red flag to come out after spinning into the gravel at Turn 5.
Following the removal of Perez' VJM07, Button's fastest lap time was soon surpassed by Nico Rosberg in his Mercedes, Ferrari's Kimi Raikkonen and Valtteri Bottas of Williams.
Marcus Ericsson's Caterham would cause the third red flag situation as the day's action winded down, but there was enough time for Button to get back out on track and top the timesheets once again.
His time of 1m24.165s on Pirelli's medium tyre would not be surpassed.
Raikkonen's 1m24.812s was good enough for second place, while Bottas was third with 1m25.344s.
Despite finishing fourth with a 1m25.588s on the normal Pirelli hard, Rosberg enjoyed a highly productive day, completing a marathon 97 laps - more than the amount completed by all the drivers involved in Tuesday's testing combined - including a final run of 24 straight laps.
Raikkonen had the highest number of laps after Rosberg, but he was still well down on the Mercedes driver's tally.
Perez managed to set the fifth fastest time before his day ended prematurely, notably much quicker than sixth-placed Esteban Gutierrez in his Sauber.
Teams with a Renault engine all struggled on the day, a suspected energy store issue the culprit. Only Caterham's Ericsson managed to get his lap count into double figures - though he could not get within 14 seconds of Button's time over his 11 completed laps.
Vettel was eighth quickest on the day, but managed only eight laps as a result of the Renault battery problem. Following the three laps they managed on Tuesday, it has not been a particularly productive test for Red Bull so far.
Of the remaining teams, Toro Rosso did not make it out on the day, while Marussia took delivery of its car towards the end of proceedings and hope to run on day three.
1 J Button McLaren 1m24.165s 43 laps
2 K Raikkonen Ferrari 1m24.812s +0.647s 47 laps
3 V Bottas Williams-Mercedes 1m25.344s +1.179s 35 laps
4 N Rosberg Mercedes 1m25.588s +1.423s 97 laps
5 S Perez Force India-Mercedes 1m28.376s +4.211s 37 laps
6 E Gutierrez Sauber-Ferrari 1m33.270s +9.105s 53 laps
7 M Ericsson Caterham-Renault 1m37.975s +13.810s 11 laps
8 S Vettel Red Bull-Renault 1m38.320s +14.155s 8 laps
Gallery: Day Two Of Testing
Rosberg Smiling After Day Two