A procedure which can serve as a possible basis for the laboratory study of the topical effect of NSAID was investigated in rats or guinea pigs. The effect of NSAID was greatly influenced by physical characteristics of the preparation such as drug particle size, solubility, ointment base and concentration of drug. Moreover, it was also found to be affected by many technical factors such as animal fixation, drug application times and methods (rubbing times or occlusive dressing technique) and amounts applied which play an important role in topical preparation. The topical application of NSAID ointment (1% of indomethacin, ketoprofen or diclofenac sodium) markedly inhibited the paw edema by carrageenin in rats. The inhibitory activity was the same as that of steroidal ointment (0.12% betamethasone 17-valerate or 0.05% fluocinonide), but was less than that by oral administration of these NSAID. Also, the NSAID ointment obviously inhibited the ultraviolet erythema in guinea pigs and the swelling in the hind feet of adjuvant arthritic rats. The inhibitory activities of NSAID ointments on these inflammatory responses were almost the same as those obtained by oral administration of such NSAID and more potent than those of steroidal ointments. Furthermore, NSAID ointments increased the pain threshold in the inflamed foot as determined by the method of Randall and Selitto. The analgesic activity of NSAID ointment was more potent than that of steroidal ointment, but less than that of NSAID administered orally. On the other hand, neither the systemic effects such as decrease in weight of the adrenals and thymus which were noted when steroidal ointment was used, nor the gastrointestinal lesions which were found by oral administration of NSAID, were recognized in rats in which NSAID ointment was applied topically. The anti-inflammatory effects of NSAID ointment correlated well with the drug concentration at the site of inflammation. These findings suggest that NSAID ointment has a clinical use in the treatment of inflammatory diseases.