2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a mutagen found in cooked meat, has been shown to induce mammary gland tumors in rats. Our laboratory recently observed that a high fat diet enhances the incidence and severity of PhIP-induced mammary gland cancer in rats. In the current study, reverse transcription followed by polymerase chain reaction amplification was used to determine whether EGFR, TGF-alpha, neu and c-myc are differentially expressed in PhIP-induced mammary gland tumors classified histologically as benign or malignant and to evaluate whether dietary fat intake influences the expression of these genes. Of 23 total PhIP-induced mammary tumors examined, 43%, 57% and 74% had increased expression of EGFR, TGF-alpha and neu mRNA respectively. Increased expression of these genes appeared to be consistently present in tumors displaying papillomatosis. In contrast, to the other three genes, c-myc mRNA levels were infrequently elevated. The percentage of dietary fat did not appear to influence the expression of EGFR, TGF-alpha or neu in either tumors or mammary gland from control rats. However, the levels of c-myc mRNA were 1.8- and 2.9-fold higher in the control mammary gland and benign PhIP-induced tumors respectively in rats fed the high-fat diet than in rats fed the low-fat diet, suggesting a slight effect of dietary fat (P < 0.08) on c-myc expression. These results suggest that increased expression of EGFR, TGF-alpha and especially neu is associated with PhIP-induced mammary gland cancer in rats.