Formula One world champion Lewis Hamilton was fastest but teenage rookie Max Verstappen stood out with an eye-catching practice performance on a rainy first day at the Monaco Grand Prix.
Hamilton, fresh from signing a new three-year deal with Mercedes, was quickest around the tight street circuit on a cool morning and then lapped in a day's best of one minute 17.192 before rain limited running in the afternoon.
The championship leader, who is out to deny team mate and title rival Nico Rosberg a third win in a row in the Mediterranean principality, completed only 12 laps after lunch as the skies opened.
If Hamilton had been expected to set the pace, Verstappen's early showing was surprising and highly impressive.
The 17-year-old, the only driver in the field never to have raced in Monaco, looked a natural as he lapped only 0.149 slower in his Toro Rosso after completing 42 near-faultless laps in the morning.
The Dutch driver, son of former racer Jos, popped in his fastest lap just before the end when the track had most grip.
He was seventh fastest in the afternoon in a session red-flagged after 14 minutes when Marussia's Robert Merhi lost control at the tunnel exit and smashed into metal barriers in the first incident of the weekend.
Steady rain then ensured all the drivers stayed in the garages as the television cameras focused on seagulls, puddles and damp fans sheltering under umbrellas.
When the rain eased, with about 10 minutes remaining, most drivers again ventured out but the leading times were unchanged.
Rosberg, 20 points behind Hamilton after five of 19 races this season, was ninth fastest in a first session that started with drivers on intermediate tyres after a heavy overnight storm swept the Riviera.
He was second in the afternoon, 0.740 slower than Hamilton, with Ferrari's Sebastian Vettel and Kimi Raikkonen third and fourth.
Rosberg brushed the barriers without consequences at Tabac in the morning and several others went down the escape road at the Ste Devote corner.
Australian Daniel Ricciardo ended that session third fastest for Red Bull, ahead of Vettel and Toro Rosso's Spanish rookie Carlos Sainz.
Britain's Jenson Button sat out the first hour while McLaren mechanics worked on his car but eventually surfaced to set the 12th best time with team mate Alonso 11th.