Several blood steroids, serum gonadotropins and cytosol estradiol receptors of the anterior pituitary and hypothalamus were quantified in immature female rats which were induced to ovulate with pregnant mare's serum gonadotropin (PMSG). Studies revealed that serum levels of progesterone, 17-hydroxyprogesterone, testosterone, androstenedione and estradiol were initially elevated at 6 PM (day 30) after administration of 8 IU of PMSG at 10 AM day 30. Serum levels of estradiol and testosterone rose progressively from day 30 through the AM of day 32. A further increase in serum concentrations of progesterone, 17-hydroxyprogesterone, androstenedione, testosterone, and dehydroepiandrosterone occurred on the PM of day 32 whereas serum estradiol levels declined. Serum levels of all steroids declined on the day of estrus (33) and only progesterone levels were further elevated on day 34 (diestrus). Dihydrotestosterone concentrations were minimally altered by PMSG treatment. Saline administration resulted in no significant alterations in levels of any steroid quantified from day 29 to 34 in control animals. A progressive decline in cytosol estradiol receptor content of the anterior pituitary and hypothalamus was documented following PMSG treatment of intact female rats; there was no depletion of receptors following PMSG administration to ovariectomized immature rats. Maximal depletion of cytosol estradiol receptors occurred on day 32 with replenishment of cytosol estradiol receptor levels on estrus (day 33). The preovulatory gonadotropin surge was found to occur on the PM of day 32 after maximal receptor depletion. The cycle of depletion and replenishment of receptors was repeated during a second spontaneous estrous cycle four days later which coincided with a rise and fall in serum estradiol levels. It is suggested that the depletion of cytosol estradiol receptors of the anterior pituitary/hypothalamic unit may be causally related to the preovulatory gonadotropin surge resulting from PMSG administration to immature female rats. In addition, changes in blood steroids and gonadotropins after PMSG treatment are similar to those reported for proestrus-estrus-diestrus I of the normal adult estrous cycle. These findings further demonstrate the validity of the PMSG-primed immature female rat preparation as a model for the estrous cycle of the adult rat.