The effects of hypothalamic deafferentation on the estrous cycle and ovulation were studied in the mature female guinea pig, a spontaneous ovulator with a true luteal phase. Animals were stereotaxically deafferentated using the Halász knife technique. (1) Large complete deafferentation of the medial basal hypothalamus (MBH; arcuate nucleus, median eminence (ME), and a ventral part of the ventromedial nucleus) did not block cyclic ovulation, although the cycle became slightly prolonged. (2) Small complete deafferentation which excluded the ventromedial nucleus from the MBH and partially damaged the arcuate nucleus and ME resulted in anovulation and constant closure of the vaginal membrane. (3) Large anterior deafferentation, which eliminated neural afferent from medial preoptic area (MPO), suprachiasmatic portion of the preoptic nucleus (POSC) and the anterior periventricular area to the MBH, was effective in blocking ovulation and inducing constant vaginal opening. In most of the cases, the knife passed rostral to the suprachiasmatic nucleus (SCN). (4) Small anterior deafferentation at the caudal border of the optic chiasma did not block ovulation if the arcuate nucleus and ME were intact, although the cycle sometimes became irregular. (5) Posterior deafferentation at the level of the mammillary bodies failed to interfere with cyclic ovulation. It is concluded that, in the guinea pig, the deafferentated MBH is capable of sustaining both tonic and phasic secretion of gonadotropins (Gns) necessary for maintenance of normal estrous cycles and ovulation. However, this function of the MBH may also be modulated by facilitatory and inhibitory influences arising from extrahypothalamic areas