Leptin, a product of the obese gene, is associated not only with obesity but also with female reproductive function, but it has not yet been ascertained whether leptin acts directly on the ovary or indirectly via the hypothalamus-pituitary pathway. Therefore, the object of this study was to investigate the expession of leptin and its receptor in the rat ovary by immunohistochemistry and RT-PCR during the estrous cycle. Immunohistochemistry results showed that leptin was stained in the theca cells and in part of granulosa cells in atretic follicles, whereas leptin receptor was localized in the interstitial cells and ova in preantral follicies. In particular, leptin and its receptor in atretic follicles displayed more intensive staining compared to those in normal follicles. During the estrous cycle, the mRNA expression of leptin and its receptor in the ovary was detected by RT-PCR and estradiol, progesterone, and leptin levels in the serum was measured by ELISA. The leptin level in the serum on metestrous phase was significantly higher than that on estrous phase. Similar to leptin level, progesterone level increased on metestrous phase. Leptin mRNA was not detected throughout the estrous cycle, whereas leptin receptor mRNA was expressed on all phases of estrous cycle excepting the diestrous phase. These results suggest that leptin might be directly involved in the regulation of ovarian function in rat.