Metronidazole (MNZ), a widely used therapeutic drug, was administered to male and female Swiss mice intragastrically at a dose of 2 mg MNZ/mouse per day, 5 days a week, every alternate week, throughout their life span to test its carcinogenicity. The treatment induced a significant increase in the overall incidence of tumors in female mice but not in male mice. At the same dose, no teratogenic effect was observed. Perinatal carcinogenicity was studied by following up animals till the end of the F2 generation. Though different groups of animals were considered, a significant increase in tumor incidence was observed only in F1 mice which received MNZ treatment during gestation as compared to the corresponding control mice which received distilled water. The F1 mice which received MNZ during gestation, lactation, and subsequently in adulthood, and the F2 progeny had a tumor incidence comparable to that observed in control mice.