Large bowel carcinoma was induced in rats by injecting MNNG intrarectally, and changes in the large bowel mucosa prostaglandin (PG) with time were determined. The PGE2 levels of the colonic mucosa in a control group were 20.9±9.7 (pg/mg total protein) at 5 weeks, 25.5 ±9.7 at 10 weeks, 26.5±18.1 at 20 weeks, and 34.8±12.7 at 40 weeks. The PGE2 levels in the MNNG-treated group were 44.7±6.2 at 5 weeks, 43.1±14.9 at 10 weeks, 70.1±23.4 at 20 weeks, and 79.7±54.1 at 40 weeks. The intrinsic PGE2 levels of the noncancerous mucosa were thus significantly higher for the MNNG group than for the control group at all weeks. At 40 weeks, the PGE2 levels of cancer lesions were significantly high compared with those of the noncancerous area. In the cancerous lesions, 6-keto-PGF1α and PGF2α decreased and TXB2 increased significantly at 40 weeks. The observations demonstrated that PGE2 was implicated as a promoter in the development and proliferation of carcinoma in MNNG-induced large bowel carcinogenesis in rats.