The rodent uterus is a widely studied target tissue for sexual steroid hormone action. The aim of the present study was to assess the molecular mechanism that participates in the initiation of cell proliferation of the rat uterine epithelial cells during the estrus (E)-metestrus (M) transition. Cell proliferation, ERalpha, c-fos, cyclin D1 and D3, cdk4, and cdk6 proteins were assessed in these animals by immunohistochemistry. Estradiol (E(2)) and progesterone (P(4)) plasma levels were assessed by RIA. The results indicate that the glandular epithelium starts to proliferate at 21:00 h on estrus day, and initiates at least 3h before the luminal epithelium does. Fos expression was markedly increased during the afternoon of estrus day, and its increase was in parallel to ERalpha expression. Interestingly, both, cyclin D1 and D3 were abundantly expressed in the luminal and glandular epithelia, and nuclear immunolabelling of cyclin D1 and D3 precedes BrdU incorporation in the cell. cdk4 and cdk6 were localized in the nuclei in both epithelia throughout the studied time course. In addition, cdk4 was more abundant throughout estrus and metestrus days than cdk6. The overall results indicate that ERalpha, Fos and cyclins D1 and D3, cdk4 and cdk6 are expressed in both glandular and luminal epithelia of the rat uterus during the E-M transition. In conclusion, there is a good correlation between sequential expression of these proteins and cell cycle progression in the rat uterine epithelial cells during the estrous cycle. However, the differences observed in the cellular localization, time course of expression and the cellular types that express both cyclins between physiological and pharmacological conditions, demonstrated different mechanisms of regulation and should be due to the complex hormonal milieu during the estrous cycle.