Abstract: AIM　To study the effects of feprazone (Fep) on cyclooxygenase (COX) activities in vitro. METHODS　Prostaglandin E₂(PGE₂) in cultured mouse peritoneal macrophage supernatants induced by lipopolysaccharide (LPS) 1 mg*L^-1 was measured with radioimmunoassay (RIA) method using ³H-labelled PGE₂. The 6-keto-PGF_1α in cultured bovine arterial endothelial cell (BAEC) supernatants was measured with RIA method using ¹²⁵I-labelled 6-keto-PGF_1α. RESULTS　Fep markedly inhibited PGE₂ production in a good concentration-dependent manner at the concentrations of 0.1, 1.0 and 10.0 μmol.L^-1. The IC₅₀ of Fep is 1.0 μmol.L^-1. Fep significantly decreased 6-keto-PGF1α production at the concentration of 10.0 μmol*L^-1. CONCLUSION　The results showed that the inhibitory effect of Fep on COX2 is 10 fold more than that on COX1. Thus, it is suggested that Fep have more potential inhibitory effect on COX2 and lower gastrointestinal toxicity than normal nonsteroidal antiinflammatory drugs such as aspirin, indomethacin and meloxicam.