Induction of mammary carcinomas in rats by a single exposure to a carcinogen during sexual development often involves malignant activation of the Ha-ras-1 locus. Each of the Ha-ras-1 oncogenes present in tumours induced by N-nitroso-N-methylurea, but not in those induced by 7,12-dimethylbenz(a)anthracene, became activated by the same G----A transition, the type of mutation induced by N-nitroso-N-methylurea. These results are consistent with the notion that Ha-ras-1 oncogenes are directly activated by the carcinogen during initiation of neoplasia.