A team spokesman said the driver, younger brother of Ferrari's six times world champion Michael, had returned home to Salzburg in Austria after seeing doctors in Valencia.
"He has seen his own doctors now and it's his lower stomach," he said. "He's been given something for it and he will be in the car again in Barcelona on Monday testing again."
Ralf, fifth overall last season, missed the Italian Grand Prix in September after suffering concussion as a result of a heavy crash in testing.
Brazilian Antonio Pizzonia, dismissed by Jaguar last season after 11 races and without a drive for this year, set the fastest time in Thursday's Valencia test with a track record time.
Pizzonia lapped the Ricardo Tormo circuit in one minute 09.161 seconds in the new Williams, with Michael Schumacher second fastest in last year's Ferrari in 1:09.412.
