We have previously reported that mouse mammary epithelial cells transformed in vitro yield tumors which vary qualitatively and quantitatively as a function of the mitogenic environment in which the cells are propagated at the time of carcinogen treatment. One milieu supportive of transformation in vitro was medium supplemented with progesterone and prolactin as the mitogens. We have performed parallel studies in which virgin mice were isografted with pituitaries resulting in elevated serum titers of progesterone and prolactin. After carcinogen treatment, these mice developed mammary tumors which included those identical genotypically and phenotypically to tumors induced in vitro in cells grown in progesterone and prolactin during carcinogen exposure. Our current working hypothesis is that the mitogenic environment around the time of carcinogen administration can modulate the incidence and phenotype of the resultant tumors. To further test this hypothesis, we have evaluated the susceptibility of hormonally-stimulated parous mice to chemically induced mammary carcinogenesis since parity is known to significantly reduce the susceptibility of the mouse mammary gland to carcinogenesis. Virgin or multiparous BALB/c mice were isografted with two pituitaries. Five weeks after surgery, the mice were injected with N-methyl-N-nitrosourea (MNU; 50 micrograms/g i.v.). Mammary carcinomas arose in 85% (11/13) with a median latency of 22.8 weeks and 1.9 tumors per virgin mouse and 80% (24/30) with a median latency of 22.1 weeks at a frequency of 1.9 tumors per parous mouse. Only 14% (2/14) of the non-isografted, age-matched parous controls developed tumors when injected with MNU. Fourteen parous mice receiving only pituitary isografts (no MNU), did not develop mammary carcinomas within the 7-month period of the study. These results demonstrate that parous BALB/c mice are refractory to MNU-induced mammary carcinogenesis and that this refractoriness is not permanent, but can be overcome by hormonal stimulation mediated by pituitary isografts.