In order to clarify the possible mechanisms underlying the inhibition of uterine tumorigenesis in rasH2 mice carrying a human prototype c-Ha-ras gene when treated with ethinylestradiol (EE), mice were given a single intraperitoneal injection of 120 mg/kg N-ethyl-N-nitrosourea (ENU), followed by 2.5 ppm EE for 6 weeks. Many genes involved in estrogen responses and cell proliferation in the uterus of rasH2 mice are activated by treatment with ENU followed by EE. These include genes associated with receptors such as estrogen receptor (ER) alpha, cell cycles such as cyclin E, cyclin dependent kinase (CDK) 4, CDK inhibitors, ubiquitin and smad3, and growth factors such as epidermal growth factor (EGF) receptor, transforming growth factor (TGF) beta, TGF beta receptor and insulin-like growth factor (IGF). In this study, the ENU+EE treated rasH2 mice demonstrated acceleration of estrogen decomposition and down-regulation of the expression of ER alpha. TGF beta, serine/threonine kinase and smad3, which are downstream of the TGF beta signaling pathway, were also down-regulated, indicating signal values such as CDK4 were down-regulated in the ENU+EE treated rasH2 mice. Ubiquitin which indicates CDK inhibitory metabolism was also down-regulated. Finally, several genes involved in growth factors such as EGF receptor, TGF beta receptor and IGF were also down-regulated. On the other hand, the ENU+EE treated ICR mice showed deceleration of estrogen decomposition and up-regulation of the expression of ER alpha, down-regulation of TGF beta, serine/threonine kinase, smad3, CDK4, ubiquitin, EGF receptor, TGF beta receptor and IGF, indicating that these genes appear to act as stimulants of cell proliferation and carcinogenesis. Therefore, we consider that these genes are key genes which are strongly involved in the inhibition of uterine carcinogenesis in ENU-initiated rasH2 mice treated with EE.