In an intial attempt to localize the central components of the neuroendocrine control systems whereby estrogens regulate gonadotropin secretion, the medial basal hypothalamus (MBH) was surgically isolated from the remainder of the brain in 18 female rhesus monkeys using a modified "Halasz knife". The completeness and localization of the resulting "cuts" were verified by examinations of serial 50 mugM coronal sections through each hypothalamus. The cuts extended from the optic chiasm to the mamillary bodies and from midline to the medial aspects of the optic tracts. The MBH "ISLANDS" ISOLATED BY THESE CUTS INCLUDED THE MEDIAN EMINENCE AND ARCUATE NUCLEI AS WELL AS PORTIONS OF THE VENTROMEDIAL NUCLEI, PREMAMILLARY AREAS AND MAMILLARY BODIES. Following complete MBH isolation, 6 of 7 otherwise intact monkeys responded to an estradiol benzoate (EB) injection with large LH and FSH surges. Three of these animals also displayed spontaneous gonadotropin surges which eventuated in ovulation as evidenced by sustained elevations in circulating progesterone levels. Similar patterns of LH and FSH secretion in response to EB administration were also observed in 6 of 7 intact animals in which only the superofrontal imputs to the MBH WERE INTERRUPTED. Four of these monkeys ovulated spontaneously. Circulating gonadotropin levels fell slightly in ovariectomized monkeys following complete MBH disconnection, but the circhoral, pulsatile rhythms of gonadotropin release persisted. These animals also responded to the negative and positive feedback actions of estrogen as evidenced by initial declines in circulating LH and FSH concentrations followed by surges of these hormones at the appropriate times following the injection of EB. It would appear from these observations that the sites of the negative and positive feedback actions of estrogen on LH and FSH secretion and, therefore, the sites of the central components of the neuroendocrine systems which control tonic and surge secretion of the gonadotropic hormones in the rhesus monkey may be resident within the MBH-hypophysial unit.