Virgin 50-55-day-old rats exhibiting regular estrous cycles were injected i.v. with the direct acting carcinogen 1-nitroso-1-methylurea (NMU) on the morning of proestrus, estrus, or diestrus. Rats were killed at weekly intervals following NMU administration to identify source and number of microscopically identifiable dysplasias. Terminal end bud (TEB) abnormalities appeared within 1 week following NMU administration, with a significantly greater number of abnormal TEBs in mammary glands of rats injected on proestrus (PE) and estrus (E) than on diestrus (DE). Ductal (DH) and ductal alveolar hyperplasias (DAH) and hyperplastic alveolar nodules (HAN) appeared during week 3, with significantly more of each type of lesion appearing by 6 weeks after NMU injection. HAN were most numerous in glands from rats injected on estrus. Adenocarcinomas arose from both the proximal and distal ductal network; at 10 and 12 weeks post NMU, significantly more tumors were found in rats injected on proestrus than diestrus and estrus. These results support the theory that the hormonal environment at the time of NMU administration significantly alters early development of mammary tumors in the rat.