Abstract: AIM: To detect cyclooxygenase-2(COX-2)/prostaglandin H synthase-2(PGHS-2) in cells of pleural exudate and investigate the effect of NS-398 on its activity. METHODS: COX-2 was tested by SDS-PAGE and Western blot in cells of the pleural exudate induced in rats by intrapleural injection of 0.2 ml of 2% carrageenin. TLC was used to analyze the activity of COX-2. RESULTS: The examination using PGHS-1 or PGHS-2 antiserum showed that PGHS-2 appeared at the 5th hour after carrageenin injection, and vanished at the 19th hour. While, PGHS-1 appeared in cells before and after carrageenin injection. Furthermore, only PGHS-1, but not PGHS-2, was detected in the microsomal fraction of the lung, stomach and kidney of pleurisy-affected and normal control rats. NS-398, a novel nonsteroidal anti-inflammatory agent, suppressed the activity of COX-2 with IC₅₀=4.5　μmol.L^-1. CONCLUSION: COX-2 is only present in inflammatory site induced by carrageenin. NS-398 may selectively inhibit COX-2 activity in a dose-dependent manner.