AIM: To study the inhibitory effects of benzisoselenothiazolidone sulfonamide derivatives on cyclooxygenase. METHODS: 6-Keto-PGF1 alpha and PGE2 were assayed by radioimmunoassay (RIA) method; mRNA expression of COX-1 and COX-2 were assayed by reverse transcription-polymerase chain reaction (RT-PCR) method. RESULTS: Compound A [N-4-(4-methoxyphenyl-aminosulfonyl)-benziososelenothiazolidone] and B [N-4-(4-fluorophenyl-aminosulfonyl)-benzoisoselenothiazolidone] were two benzoisoselenothiazolidone sulfonamide derivatives, which can inhibit COX activity with IC50 of 1.5 x 10(-8) mol.L-1 and 5.0 x 10(-8) mol.L-1 for COX-2, as well as IC50 of 1.5 x 10(-5) mol.L-1 and 2.8 x 10(-5) mol.L-1 for COX-1. The ratio of IC50 COX-1/IC50 COX-2 of compound A and B are 1,000 and 560, respectively. They both can inhibit COX-2 mRNA expression in cultured rat peritoneal macrophages stimulated with LPS (1 microgram.mL-1), and have no effect on COX-1 mRNA expressions. CONCLUSION: Compound A and B, two benzoisoselenothiazolidone sulfonamide derivatives, both are selective inhibitory agents of COX-2, and possess inhibitory effects on 5-lipoxygenase and cyclooxygenase.