Williams' new Formula One car has made a promising debut, setting the pace on its first run alongside other teams at Jerez on Wednesday.
Montoya had a promising first day's running in the FW26
Juan Pablo Montoya set the fastest lap at Spanish track, more than a second quicker than Toyota's Ricardo Zonta.
Williams were the only team at the test running a 2004 model.
Williams chief operations engineer Sam Michael said: "We have encountered one or two teething problems, but Juan managed to complete some good mileage."
Montoya set a best time of one minute 16.630 seconds, and completed 51 laps.
"We've concentrated on trying a variety of different set ups
and collecting initial data from the car," said Michael, adding that the problems were to be expected of a new car.
Renault and Ferrari were also at the test.
Renault driver Jarno Trulli was third fastest, just over 0.1secs slower than Zonta, followed by Williams test driver Marc Gene in a 2003 car.
Renault test driver Franck Montagny was fifth quickest, ahead of the Ferraris of new Sauber driver Felipe Massa and test driver Luca Badoer.