We investigated the concept that the enhanced development of DMBA-induced mammary adenocarcinomas in rats fed a high fat diet might be mediated by prolactin via the hypothalamic-pituitary system rather than by a direct effect of a fat on the mammary gland itself. Prolactin levels in serum from female Sprague-Dawley rats fed high (20%) and low (0.5%) lard diets for 2 and 5 mo were measured by radioimmunoassay. The levels in rats at proestrus-estrus on high-lard diets were significantly higher (250 ng/ml) than those on a low lard diet (100 ng/ml). At metestrus-diestrus prolactin levels were approximately 50 ng/ml regardless of diet. Thus, the tumor-enhancing effect of a high fat diet may be indirect and mediated by neuro-hormonal changes in the hypothalamic centers controlling prolactin synthesis and secretion.