The present study was performed to clarify the role of the ovarian carbonyl reductase (OCR) in ovarian function in immature rats. The OCR activities towards three specific substrates, 13,14-dihydro-PGF2 alpha, 4-benzoylpyridine and menadione, were photometrically and radiochemically determined in the 9,000 x g supernatants of ovaries, and OCR content was measured by Western-blot-peroxidase anti-peroxidase (PAP) analysis. Immunohistochemical localization of the enzyme in the ovary was performed by the avidin-biotin-complex (ABC) method for paraffin sections. Positive immunoreactivity with OCR antibody was observed for the theca cells and interstitial gland cells at 72 hr after pregnant mare serum gonadotropin (PMSG) treatment when ovulation was confirmed, and the granulosa cells were consistently negatively stained. The OCR activity was significantly increased by PMSG, human chorionic gonadotropin (hCG) and PMSG-hCG treatments, but estradiol and tamoxifen overcame the effect of PMSG on the enzyme activity. Moreover, estradiol enhanced the effect of hCG, but tamoxifen did not. Changes in the OCR activity well-correlated with those in the OCR content. These findings indicate that the OCR is regulated by gonadotropin and estrogen and that metabolites formed by the enzyme could be closely involved in ovarian cell function.