A surgical procedure is described for isolating the pituitary gland from hypothalamic influences in sheep. The procedure results in total deafferentation of the stalk and median eminence but maintains the blood supply to the pituitary gland. The median eminence, pituitary stalk and anterior face of the pituitary gland were exposed by a transnasal, transsphenoidal approach. In early studies section of the pituitary stalk as close as possible to the pituitary gland caused almost total infarction of the gland. Attempts were made to disconnect the pituitary gland from the hypothalamus by section immediately above the lateral inputs of the superior hypophyseal arteries but variable results were obtained, always with infarction of part of the pars distalis. When the pituitary gland was disconnected from the hypothalamus by entering the median eminence above the portal circulation and evacuating the nervous tissue of the tuber cinereum, (hypothalamo-pituitary disconnection; HPD), a small area of infarction was found in the pars distalis of only 1/10 cases. HPD effectively disconnected the pituitary gland from hypothalamic control whilst the pars distalis was not deprived of its blood supply. Complete severence of hypothalamo-pituitary connections also caused atrophy of the pars nervosa and enlargement of the cells of the pars intermedia. Following HPD, plasma LH and FSH concentrations diminished and plasma prolactin concentrations rose. On the day after surgery there were no LH, FSH or prolactin responses to 50 μg (i.m.) of oestradiol benzoate indicating the functional isolation of the pituitary gland from the hypothalamus. The isolated pituitaries were capable of responding to gonadotropin releasing hormone by LH release. Disconnection of the pituitary gland from the hypothalamus by subpial deafferentation provides a good in vivo isolated pituitary model in the sheep.