Alfa Romeo's Valtteri Bottas headed Mercedes' Lewis Hamilton in an inconclusive wet first practice session at the British Grand Prix.
They were the only two drivers to set a lap time on dry-weather tyres at the before it was curtailed by a red flag caused by Aston Martin's Lance Stroll.
The Canadian lost control on slick tyres on the damp track at Copse and beached the car in the gravel without major damage.
Only 10 drivers set a lap time.
On wet tyres, Ferrari's Carlos Sainz set the fastest time, from Bottas and the Spaniard's team-mate Charles Leclerc, all lapping at different times so their laps could not be compared.
The wet conditions meant teams were unable to judge their new upgrades.
- British GP coverage on the BBC
- Land Norris column: 'I feel bad saying no to weddings'
- Full practice results
Several teams have taken significant technical developments to Silverstone as they seek to improve their performance.
Mercedes have extensive revisions to their so-far recalcitrant W13 car, centring on a new floor design that they hope will limit the bouncing that has afflicted the car so far this season and restricted its pace.
Mercedes also expect the track characteristics at Silverstone to help them make up some ground on front-runners Red Bull and Ferrari. The surface is smooth, which should limit their bouncing problems, and there are few slow corners to exacerbate the car's other big weakness.
Red Bull and Ferrari, however, also have upgrades for the weekend - both have a new engine cover aimed at improving airflow to the rear and increasing downforce.
Williams, the slowest team on the grid, have perhaps the biggest upgrade, with Red Bull-inspired side pods, while Alpine have a new floor as well as redesigned bodywork.