Dietary lipids have a role in the aetiology of breast cancer, acting at several cellular levels. We investigated the effects of a high corn oil and a high extra virgin olive oil diet on the clinical and histopathological characteristics of rat dimethylbenz(α)anthracene-induced mammary carcinogenesis and on the expression of p21Ha-Ras, detected by immunohistochemistry, in one experimental series including a low-fat corn oil diet (LFCO) and two high-fat diet groups: HFCO(P), rich in corn oil, and HFOO(P), rich in extra virgin olive oil. Whereas the high corn oil diet tended to reduce latency time, to raise tumour incidence and to increase total tumour yield, the high extra virgin olive oil diet led to a latency time similar to that of LFCO and to a lower tumour incidence than HFCO(P) and lower total tumour yield, even than LFCO. HFCO(P) tumours displayed a higher histological grade and profile than LFCO tumours, while adenocarcinomas in HFOO(P) were similar to LFCO ones. Although no significant differences in p21Ha-Ras expression among dietary groups was found, we detected a significant p21Ha-Ras decreasing expression as grade increased, in groups LFCO and HFCO(P). HFOO(P) tumours exhibited a higher staining in high-grade carcinomas compared to the similar malignant tumours of the two other dietary groups. These data suggest that dietary lipids influence the clinical behaviour and the morphological malignancy of the experimental mammary carcinogenesis, according to the type of fat, without altering p21Ha-Ras expression. Nevertheless, this expression could be affected by the malignancy of tumours, probably through a post-translational event.