Background In vitro fertilization (IVF) researches have suggested that cystathionine β synthase (CBS) is involved in oocyte development. However, little is known about the regional and cellular expression patterns of CBS in the ovary. The purpose of this study was to analyze the localization of CBS in mice ovaries and to investigate the expression profile during follicular development. Methods We used in situ hybridization and immunohistochemical analysis to determine CBS expression in the ovaries of female Balb/c mice. Then the follicles were collected from F1 (C57BL×Balb/c) mice and cultured in vitro. With the method of semi-quantitative RT-PCR, we also investigated the expression profile of CBS during follicular development. Results CBS was absent in the oocytes, although it was ubiquitously expressed in the ovary with the strongest expression in follicular cells at all stages. In late antral follicles, CBS expression was markedly higher in granulosa cells located close to the antrum and in cumulus cells around the oocyte. The semi-quantitative RT-PCR showed that CBS mRNA was detected in follicles at all stages in vitro. In cumulus-oocyte complexes superovulated, CBS expression also increased rapidly. Conclusions CBS was located mainly in the follicular cells in the ovaries. The level of CBS expression is high in follicles during folliculogenesis in mice. Differences in the CBS expression profile between oocyte and follicular cells suggest a role for CBS as a mediator in interactions between oocyte and granulosa cells. Background In vitro fertilization (IVF) researches have suggested that cystathionine β synthase (CBS) is involved in oocyte development. However, little is known about the regional and cellular expression patterns of CBS in the ovary. The purpose of this study was to analyze the localization of CBS in mice ovaries and to investigate the expression profile during follicular development. Methods We used in situ hybridization and immunohistochemical analysis to determine CBS expression in the ovaries of female Balb / c mice. Then the follicles were collected from F1 (C57BL × Balb / c) mice and cultured in vitro. With the method of semi-quantitative RT-PCR, we also investigated the expression profile of CBS during follicular development. Results CBS was absent in the oocytes, although it was ubiquitously expressed in the ovary with the strongest expression in follicular cells at all stages. In late antral follicles, CBS expression was markedly higher in granulosa cells located close to the ant rum and in cumulus cells around the oocyte. The semi-quantitative RT-PCR showed that CBS mRNA was detected in follicles at all stages in vitro. In cumulus-oocyte complexes superovulated, CBS expression also increased rapidly. The level of CBS expression is high in follicles during folliculogenesis in mice. Differences in the CBS expression profile between oocyte and follicular cells suggest a role for CBS as a mediator in interactions between oocyte and granulosa cells.