Ferrari on top in the wet
World champion Schumacher, who managed just fourth fastest time in Friday's dry sessions, finished with a time of 1:40.540, more than 15 seconds slower than the best time of the previous day thanks to the sodden conditions.
He was in a class of his own as he finished 1.393 seconds ahead of Barrichello in the first Ferrari one-two of the year with closest challenger Kimi Raikkonen, of McLaren, 2.986 seconds back in third.
Only 12 of the 20 runners ventured out for a full lap in the session and that allowed Jordan driver Narain Karthikeyan, the first Indian in Formula One, to finish fourth fastest with a time of 1:45.641.
Ralf Schumacher was fifth in his Toyota, Takuma Sato sixth for BAR-Honda, Scot David Coulthard seventh in his Red Bull and Christijan Albers, in the 2005-spec Minardi, in eighth.
Minardi, who modified their cars to meet the 2005 regulations overnight, hit the track as soon as the lights went out to start the first 45-minute session with Austrian Patrick Friesacher the first driver out.
The circuit had been soaked in overnight rain, increasing lap times significantly, and the Austrian driver set the first time of the day with a 2:02.126-seconds installation lap to check the new bodywork.
Albers did the same, then went five seconds faster and continued to improve to set a time of 1:48.566 seconds, but Raikkonen soon went faster with a time five seconds quicker.
Australian Mark Webber continued to struggle on his debut for Williams and suffered two spins, the first at turn 13 early on and the second a high-speed spin at turn 11. He recovered from both.
Albers also had a spin but continued at turn 11 and Karthikeyan failed to finish the session when he spun twice in the same lap, first and turn nine and finally at turn 12.
Italian Jarno Trulli, in the second Toyota, and Briton Jenson Button, of BAR-Honda, completed the top ten but Friesacher had a disaster in the closing seconds when he went off heavily at turn one.
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