The relationship between pituitary activity and oocyte maturation was examined in Oryzias latipes (medaka), which has a circadian cycle of oviposition. Throughout the circadian cycle of oviposition, females possessed a population of large oocytes more than 800 micronmeter in diameter that could mature in the presence of gonadotropin. Oocyte maturation was observed in vitro in females hypophysectomized between three and ten hours after the beginning of the light period with the number of maturing oocytes increasing as hypophysectomy was delayed. Although in vivo oocyte maturation was blocked by hypophysectomy within two hours after the beginning of the light period, it was restored by a single injection of synthetic or mammalian pituitary hormones (gonadotropic, corticotropic and thyrotropic hormones) within ten hours after hypophysectomy. Of these pituitary hormones, FSH, LH and TSH could induce in vitro maturation of isolated oocytes. Oocytes matured in vitro in the absence of exogeneous hormones if they were isolated nine or more hours after the onset of light. The present study indicates that the circadian cycle of maturation of Oryzias oocytes is controlled by the release of pituitary hormone between three and nine hours after the beginning of the light period.