Mercedes drivers Lewis Hamilton and Nico Rosberg headed first practice at the Monaco Grand Prix.
Hamilton edged Rosberg by 0.032secs. The Red Bull of Daniel Ricciardo was only 0.203secs further back in third, ahead of Fernando Alonso's Ferrari.
Ricciardo was on an even quicker lap, which would have challenged the Mercedes drivers, but hit traffic.
Red Bull's Sebastian Vettel was fifth, 0.537secs off Ricciardo, and Kimi Raikkonen's Ferrari sixth.
The Finn was the same margin slower than team-mate Alonso as Vettel was off Ricciardo. Alonso locked his left front and went straight on without damaging his car at Mirabeau late in the session.
The Spaniard joined Sauber drivers Adrian Sutil and Esteban Gutierrez and Marussia's Max Chilton among those who had off-track moments.
Sutil and Chilton both spun at Mirabeau, with the Sauber sustaining light damage. Chilton's session ended after just seven laps following his spin, while Gutierrez went straight on at the first corner, Sainte Devote.
Williams' Valtteri Bottas was seventh fastest ahead of Force India's Sergio Perez, McLaren's Kevin Magnussen, Force India's Nico Hulkenberg and the second McLaren of Jenson Button.
It was an especially impressive performance from Magnussen, who was 0.244secs quicker than Button on his first experience of an F1 car around the demanding Monaco street track.
BBC F1 analyst Allan McNish said he thought Red Bull might be able to challenge Mercedes in Monaco.
"Ricciardo was on a lap that would have put him right up with Hamilton before coming across traffic," McNish said.
"Watching trackside at Tabac and the Swimming Pool, the Mercedes look very good, very supple. They absorb the bumps and kerbs very well.
"The Red Bull looks much harsher, it jumps around. But when it lands it lands with grip.
"Between Hamilton and Rosberg, Rosberg's inputs are more subtle, more gentle. Hamilton seems to have to work harder to get the lap time. Alonso had the least input possible, very subtle, quiet, calm. The Ferrari is not as quick, though."
The second session starts at 13:00 BST, with rain predicted for later in the afternoon.