The purpose of this paper is to explore the contribution of isoforms of cyclooxygenase (COX) to chronic inflammation in DBA/1J mice with type II collagen-induced arthritis (CIA). To address this question pharmacologically, we tested the effects of selective inhibitors of COX-1 and COX-2 on paw edema and the formation of arachidonic acid metabolites in the inflamed paws immunized with type II collagen (CII). Oral administration of FR140423 (3-(difluoromethyl)-1-(4-methoxyphenyl)-5-[4-(methylsulfinyl)phenyl]pyrazole), a selective inhibitor of COX-2, showed a dose-dependent anti-inflammatory effect in mouse CIA with ED(50) value of 0.20mg/kg. Indomethacin, a non-selective inhibitor of COX, also inhibited paw edema in this arthritic model. In contrast, the selective COX-1 inhibitors, FR122047 (1-[(4,5-bis(4-methoxyphenyl)-2-thiazoyl)carbonyl]-4-methylpiperazine hydrochloride) and SC-560 (5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole), had no effect in mouse CIA model. The increase of prostaglandin (PG) E(2) and thromboxane (TX) B(2) in the mouse inflamed paws was associated with the development of paw edema induced by CII. FR140423 dose dependently inhibited the levels of PGE(2) and TXB(2) in the CIA mouse paws with ED(50) values of 0.20 and 0.12 mg/kg, respectively, similar to indomethacin. In contrast, FR122047 and SC-560 had no effect. These results suggest that COX-2, but not COX-1, contributes to the edema and the formation of PGE(2) and TXB(2) in mouse CIA model.