Fourteen adult female rhesus monkeys, 8 with regular menstrual cycles and 6 ovariectomized, where subjected to hypothalamic disconnection using a modified Halasz-type knife. Complete islands were produced in 5 animals. In 4 of these, the island included the medial basal hypothalamus and the suprachiasmatic nucleus and its immediately adjacent preoptic area. None of the animals developed diabetes insipidus. The animals with complete islands doubled their body weights within 10 mo. Four of these continued to show either spontaneous or estrogen-induced LH [luteinizing hormone] surges. An increase of serum estrogen-stimulated neurophysin concentrations accompanying the LH surge occurred in all animals. There was a loss in diurnal variation of cortisol in all monkeys. Basal prolactin levels were elevated in only 1 animal who had damage to the medial basal hypothalamic area. Although the prolactin response to TRH [thyrotropin releasing hormone] was not affected by the procedure, it was diminished in the ovariectomized animals. Growth hormone release after insulin and 2-deoxyglucose was suppressed in all animals with complete disconnection. Animals with lesions in the fornix and/or posterior hypothalamus responded to insulin but failed to respond to 2-deoxyglucose. Immunocytochemical studies revealed extensive deposits of neurophysins, oxytocin and vasopressin in nerve fibers around the neoformed blood vessels of the hypothalamic scar, indicating that magnocellular elements may regenerate.