Sloan-Kettering virus gene product (Ski) is an unique nuclear pro-oncoprotein and belongs to the ski/sno proto-oncogene family. Ski plays multiple roles in a variety of cell types, it can induce both oncogenic transformation and terminal muscle differentiation when expressed at high levels. Ski/SnoN are important transcription regulators of the transforming growth factor-β (TGF-β) superfamily and function mainly through heterodimers. Since TGF-β superfamily are key regulators of follicle development and it has been previously shown that SnoN is also vital to follicle development, this research was conducted to clarify the relationship between Ski expression and mouse follicular development, in ovaries of neonatal and gonadotropin-induced immature mice by immunohistochemical and real-time PCR techniques. In postnatal mice, positive staining for Ski was highly detected in oocyte nuclei at postnatal day 1. With follicular development, the localization moved gradually from oocyte nuclei to perinuclear space and the total levels decreased. During the estrous cycle, Ski expression was apparent at proestrus and estrus, faint at metestrus, highest at diestrus. After injection of gonadotropin, Ski was found in perinuclear space and weak in oocyte nuclei. Following the initiation of luteinization, the expression of Ski was found in corpus luteum. Real-time PCR results also showed that Ski mRNA expression was opposite to ovulation-related genes during the cumulus expansion, with the development of the follicles, its expression level decreased. Ski is expressed in a specific manner during follicle development, ovulation and luteinization. So Ski might play essential roles in these processes especially during early follicular development.