Effects of pregnancy and lactation on a food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary carcinogenesis were investigated in female Sprague-Dawley rats. When rats were administrated PhIP in the diet (100 ppm) for 15 weeks, palpable subcutaneous tumors were first detected at week 24 in three nulliparous rats, while tumor latency was apparently increased in animals undergoing delivery and weaning during the PhIP administration period. However, since the incidence of palpable tumors was not consistently increased in the nulliparous animals, possibly due to the short PhIP-treatment, and the fact that spontaneous tumors developed in parous animals after week 44, the final incidences and multiplicities of histopathologically confirmed adenocarcinomas at week 48 were not significantly different between the two groups. These findings art partly supported by the epidemiological evidence that breast cancer is more prevalent in nulliparous than in parous women.