Prostaglandin E2, which is produced by cyclooxygenase (COX) during arachidonic acid metabolism, is considered to be related to colon carcinogenesis. Therefore, the effect of NS-398 (N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide), a COX-2 inhibitor, was examined in azoxymethane (AOM)-induced colon carcinogenesis in rats in this study. In the first experiment, groups 1-3 were treated with AOM (15 mg/kg, s.c.) 3 times at intervals of a week from 5 weeks of age. Groups 2 and 3 were respectively given 1 mg/kg and 10 mg/kg of NS-398 in 5% gum arabic aqueous solution 3 times per week by oral gavage during the experiment. Six weeks after the first exposure to AOM, aberrant crypt foci (ACF) were counted in the colonic mucosa of all rats. The mean occurrence of ACF per length in rats given 1 mg/kg b.w. or 10 mg/kg b.w. of NS-398 was reduced to 65.7% or 52.8%, respectively, of that in rats treated with only AOM. Levels of COX-2 mRNA expression in groups treated with AOM, regardless of NS-398, were slightly higher than that in the group treated with NS-398 alone as judged from reverse transcription-polymerase chain reaction analysis. In the second experiment, the effect of NS-398 at different times, i.e., during initiation and post-initiation, was examined. Treatment with NS-398 in both phases significantly inhibited the appearance of ACF. The results imply that NS-398 might have a chemopreventive potential.