2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a suspected human breast carcinogen found in cooked meat that induces mammary gland cancer in rats. By real time PCR analysis, PhIP-induced rat mammary gland carcinomas showed statistically higher expression of the G(1)-S cyclin D1 (5-fold) and its kinase partner cyclin-dependent kinase (Cdk)-4 (37-fold) in comparison with normal mammary gland, whereas cyclin D2, cyclin D3, and Cdk6 were not statistically changed. Amplification of cyclin D1 was observed by real time PCR in 24% of carcinomas (15 of 63). Only 1 of 47 carcinomas showed Cdk4 amplification. By Western blotting, the level of phospho-Rb was >2-fold higher in carcinomas than in normal mammary gland. By immunohistochemical analysis, cyclin D1, Cdk4, and phospho-Rb nuclear protein expression was 5.7-, 3.9-, and 2.3-fold higher, respectively, in carcinomas than in normal mammary gland, whereas the expression of cyclin D2, cyclin D3, and Cdk6 was similar. Among carcinomas, Cdk4 and phospho-Rb levels were positively correlated with cell proliferation. Previous studies by this laboratory indicated that these carcinomas harbor a high frequency of H-ras mutations. The H-ras pathway is linked to the cell cycle via cyclin D1. The results from the current study implicate cyclin D1/Cdk4, phospho-Rb as a central pathway in PhIP-induced rat mammary gland carcinogenesis.