Prost, who retired after winning the last of his four world titles in 1993, tested various suspension systems but continued to be non-committal over his return to Formula One.
Prost, 40, clocked several laps in about 1min 32sec - considerably slower than laps of 1:28.5 recorded by the Williams driver Damon Hill, who was also running tests for his team.
During his early laps, Prost stopped several times to be refitted into the cockpit and on one lap his headrest broke free and sailed on to the track. A McLaren spokeswoman said: "He felt fine in the car," after Prost had driven 58 laps, and added: "He's looking forward to his second day (today)."
Prost took two days of "comfort runs" at Silverstone last month after McLaren announced he would take part in the team's development and testing. McLaren are to announce their 1996 driving team on 1 October: this year's drivers are Finland's Mika Hakkinen and Britain's Mark Blundell.
Prost won three of his four driver's championships with McLaren.