Normal cycling virgin female Sprague-Dawley rats were divided into different treatment groups and subjected to half ad libitum feed intake for 2 or 4 weeks at consecutive periods of time before and/or after administration of 7,12-dimethylbenz(a)anthracene (DMBA) and then were returned to full-feed. All rats at the end of their respective under-feeding periods showed reductions in serum prolactin and cessation of estrous cycles. However, only rats underfed 1 week prior to and 1 week after DMBA administration showed significant reductions in mammary tumorigenesis or the entire 21 weeks of the experiment when compared to full-feed controls. Rat groups underfed during subsequent weeks after DMBA administration showed no alterations in mammary tumor development as compared to full-fed controls. These results demonstrate that inhibition and perhaps permanent suppression of mammary tumorigenesis occurred only in rats underfed 1 week before and 1 week after DMBA administration. The inhibitory effects produced by this under-feeding regimen on mammary tumorigenesis may be mediated by suppression of prolactin and estrogen secretion.