Following 7,12-dimethylbenz[a]anthracene (DMBA) administration, increased tumor incidence and tumor yield were observed in intact rats fed a 20% corn oil diet compared to those fed a 0.5% corn oil diet. Elevated serum prolactin levels (determined at proestrus) were also found in the former group of animals. In order to delineate whether this was responsible for the promoting effect of dietary fat in DMBA-induced mammary carcinogenesis, rats fed these two diets were subjected to electrolytic lesion of the median eminence that resulted in higher circulating prolactin concentrations. Sham-operated animals were used as controls. Results showed that this endocrine manipulation increased the tumor incidence in the low-fat group nearly 3-fold, but it failed to elicit further enhancement in the high-fat rats. Although the serum prolactin level in the low-fat-lesioned rats was comparable to that in the high-fat-lesioned rats, the tumor incidence in the former group still lagged behind that in the latter (33.3 versus 70.4%). Thus, it can be concluded that, although increased circulating prolactin in rats fed the high-fat diet may be partly responsible for the higher tumor incidence, other factors besides prolactin may be involved in the promoting effect of dietary fat in mammary carcinogenesis.