Plasma indomethacin levels have been compared in 10 subjects following 100 mg of indomethacin from two different formulations, with similar disintegration and dissolution profiles. In four of these ten subjects plasma indomethacin levels were estimated after pretreatment with, and concurrent administration of, a buffered aspirin. The percentage of protein binding of indomethacin in the presence of salicylate was also estimated. No significant differences between peak plasma indomethacin levels with or without buffered aspirin were detected, but the rate of indomethacin absorption as shown by plasma levels, was significantly increased by pretreatment with and concurrent administration of, buffered aspirin. This was associated with a marked increase in side effects.