d-Limonene was found to be effective in reducing the average number of rat mammary carcinomas that developed in 7,12-dimethylbenz[a]anthracene-treated rats when the terpene was fed during the initiation or during the promotion/progression stage of carcinogenesis. The time to the appearance of the first tumor was extended only when d-limonene was fed during the initiation stage. These effects could not be attributed to changes in mammary-relevant endocrine functions.