Pharmacological activities of mofezolac were investigated in experimental animal models and compared with those of indomethacin, ibuprofen, mefenamic acid, aspirin and aminopyrine. Mofezolac showed a potent suppression of various writhing models in mice or rats; and its potency was slightly lower than that of indomethacin, but was higher than those of the other reference drugs. Thus, mofezolac was especially active against chemically induced writhing and also in the phenylquinone induced intraperitoneal dye leakage reaction in mice. Mofezolac also has a potent inhibitory activity on the algesic responses induced by the mechanical stimulus of the inflammed tissue. Mofezolac exhibited a therapeutic effect comparable to indomethacin in the urate synovitis in dogs. Considering the anti-inflammatory and antipyretic actions, mofezolac was obviously less effective than indomethacin, and its potency was similar to that of ibuprofen. The ulcerogenic effect of mofezolac on the gastric mucosa was far weaker than that of indomethacin. In in vitro studies, the prostaglandin biosynthesis and platelet aggregation were inhibited to the same extent by both mofezolac and indomethacin. Accordingly, it may be considered that the actions of mofezolac are due to the inhibition of cyclooxygenase. Our result suggest that mofezolac can be a useful drug that shows a rapid pain-relieving activity in acute inflammations.