Somatostatin (GHIF), when administered as a 1-hr infusion (500 μg/hr) to 5 patients with Nelson's Syndrome, resulted in a sustained, progressive fall in plasma ACTH in each patient to 40% to 71%of basal values with a return toward initial levels after cessation of the infusion. The mean reduction in plasma ACTH was 48% (p < 0.005). These findings suggest that GHIF receptors not functional or present in normal pituitary tissue are present in ACTH-producing pituitary tumors.