GP3 Series championship leader Mitch Evans laid down a marker in the category's first ever session in Monaco by setting a fastest free practice time seven tenths clear of his nearest rival.
The times at the top were close for much of the session, with Kevin Ceccon (the only driver with experience of the track after racing there in GP2 last year) setting the pace early on.
Title favourite Evans moved ahead in the closing stages though to overcome Antonio Felix Da Costa and Conor Daly. Kiwi ace Evans finished up 0.715 seconds clear of Portuguese driver Felix Da Costa, with Barcelona Race 2 winner Daly.
The session was red flagged with two minutes remaining after Will Buller spun and stopped on track. The Northern Irishman ended up 14th, after running second to Carlin teammate Felix Da Costa early on.
Fourth was Aaro Vainio, who has previous in the Principality as winner of the Monaco Kart Cup in 2008. Ceccon finished up fifth, but 1.156s away from Evans.
Alex Brundle was a strong sixth ahead of David Fumanelli, Daniel Abt, Matias Laine and Marlon Stockinger.
It's an early start for the drivers on Friday, with qualifying taking place at 06:40 BST. Qualifying will be split into two groups for the odd and even numbered cars.