The P138 raced twice in 1968 but retired from both races due to engine failure, with Mexican Pedro Rodríguez at round nine in Italy and American Bobby Unser at round eleven in the United States.

[5] The Monaco Grand Prix saw Surtees retire with a broken gearbox due to him missing a gear and the Brabham of Jack Brabham crashing into the back of Surtees.

The P138 was not used in Britain because Surtees drove the P139 but the P138 reappeared at the German Grand Prix with Jackie Oliver driving but retired with an oil leak.

[8] The P138 was not used in Italy because Oliver had a second P139 to drive but the P138 reappeared at the Canadian Grand Prix with Canadian Bill Brack driving but was ten laps down and not classified.

[9] Brack was replaced by Canadian George Eaton but retired from the final two races of 1969, engine failure in the United States and a broken gearbox in Mexico.