Serum concentrations of LH, FSH, prolactin, estradiol, and progesterone were determined in female rats throughout the estrous cycle, ovariectomized rats, male rats, and female rats after surgical interruption of anterior or all afferent fibers to the medial basal hypothalamus (MBH). In control females, estradiol levels were lowest at estrus and rose steadily to peak on the afternoon of proestrus; pro-gesterone was high during the first day of diestrus and also showed a large peak on the afternoon of proestrus, at which time all three pituitary hormones were elevated. The proestrous steroid peaks occurred after the LH peak.Serum levels of all the hormones except prolactin were markedly reduced in rats with no afferent input to the MBH, whereas animals in which anterior fibers to the MBH were transected were found to have elevated levels of LH, prolactin, and estradiol, normal FSH values, and reduced progesterone. Analysis of 2 h integrated samples in rats lacking anterior input to the MBH revealed a slight reduction in LH and estradiol release after the first collection period. Prolactin values generally were highest during the first and last collection periods in individual rats, and progesterone levels were extremely high, suggesting a significant adrenal contribution.