Although discovered as an exogenous agent of mammary carcinoma, the mouse mammary tumor virus (MMTV) is also transmitted vertically as endogenous proviral DNA present in the germ line of all inbred mice. In the C3Hf mouse, which receives no exogenous virus, the initial event of mammary tumorigenesis is the transcription of the endogenous MMTV proviral DNA present at the Mtv-1 locus. Transcription occurs as a result of the hormonal effects of pregnancy, and Mtv-1 specific transcripts are seen in second-parity lactating mammary glands of these mice. As a means of studying the effects of diet on mammary carcinoma at the molecular/genetic level, we have studied the transcription of the Mtv-1 locus in C3Hf mice on a high-fat diet containing 46% fat in calories or a low-fat diet containing 10% fat in calories. We have detected an accelerated transcription of the Mtv-1 locus (first- vs. second-parity lactating mammary glands) in > 50% of the C3Hf mice on the high-fat diet. In addition, mice on the high-fat diet developed mammary tumors earlier (11 vs. 17.8 mos) and after fewer litters (2.1 vs. 4.2). Our results indicate that fat in the diet can affect gene expression related to mammary carcinoma.