The effects of vasoactive intestinal polypeptide (VIP), TRH, dopamine, and rat median eminence extract on GH release from GH-secreting pituitary adenomas were studied in vitro using a sensitive superfusion method. Dispersed pituitary tumor cells obtained from three patients with acromegaly were placed in a superfusion column, and the amounts of GH in the superfusate were determined. The addition of VIP (10(-6) M) to the perfusion system resulted in a marked increase in GH release in all three cases, and a dose-response relationship in VIP (10(-8) 10(-6) M) induced GH secretion was observed in one case studied. TRH (10(-7) M) and median eminence extract (1 equivalent/ml) also caused an abrupt and marked increase in GH release in all of the experiments. The infusion of either dopamine (10(-7) M) or bromocriptine (10(-7) M) inhibited GH secretion. These results suggest that VIP as well as TRH stimulate GH secretion by a direct action on GH-secreting pituitary tumor cells in at least some acromegalic patients.