The plasma levels of ibuprofen were measured in five healthy subjects who took 600 mg tablets of ibuprofen twice daily, three times daily and four times daily in a crossover study. Peak plasma levels were obtained 1 hour after the first dose in all but one subject (slow absorber), the mean peak value being 51.3 micrograms X ml-1 (range 39.4-63.7 micrograms X ml-1). After the repeated dose regimens of two, three or four times daily of ibuprofen, the peak levels achieved were in a similar range to those seen after the first dose: Twice daily 39.4-66.4 micrograms X ml-1; Three times daily 43.6-63.3 micrograms X ml-1; Four times daily 44.1-58.4 micrograms X ml-1. There was no evidence of accumulation of the drug and no side-effects occurred during the trial.