In this study we have investigated the influence of dietary mono- and polyunsaturated lipids on 7,12-dimethylbenz(alpha)anthracene (DMBA)-induced mammary tumorigenesis, and the modulation of expression of c-erbB1/EGFR and c-erbB2/neu as a mechanism of this influence. Two series of Sprague-Dawley rats were given a single dose of DMBA. Animals from Series 1 were fed a semi-synthetic low-fat or high-fat corn oil diet, following an initiation-promotion experimental design. Rats from Series 2 were fed low-fat or different high-fat diets (rich in corn oil or rich in olive oil) in the promotion stage of mammary carcinogenesis. High corn oil diet showed clearly a stimulatory effect on experimental breast carcinogenesis, suggesting a role of this kind of lipids on initiation and promotion of mammary tumorigenesis. Moreover, results suggested that olive oil acts as a negative modulator of the experimental breast cancer. On the other hand, two transcripts of EGFR and one neu mRNA were detected by chemiluminescent Northern blot in mammary adenocarcinomas. Analysis of data showed the tendency that high-fat corn oil diet decreases the 2.7 kb mRNA of EGFR, encoding a truncated form of the receptor with no enzimatic activity. High-fat olive oil diet increased EGFR mRNA levels, specially those from 2.7 kb, and decreased the relative abundance of neu mRNA. These data suggest that the modulating effect of dietary lipids on mammary carcinogenesis could result in changes of EGFR and neu mRNA, leading to an increase of EGFR activity by high-fat corn oil diet and a decrease of EGFR and Neu signal transduction pathway by high-fat olive oil diet.