The effects of several drugs on the release of human growth hormone (hGH) were investigated, and the following results were obtained. 1). Single intravenous injection of insulin or drop infusion of L-arginine provocated an increase in plasma hGH in 3 out of 8 and 4 out of 6 patients with active acromegaly, respectively. 2). Single intravenous injection of thyrotropin-releasing hormone (TRH) or luteinizing hormone-releasing hormone (LH-RH) provoked a prompt increase in plasma hGH in 7 out of 13 and 4 out of 10 patients with acromegaly, respectively, whereas no significant change in plasma hGH was observed in normal subjects. 3). Plasma hGH levels decreased significantly within 30 min after an injection of glucagon in 7 out of 10 patients with acromegaly. 4). Single oral administration of L-dopa or 2-Br-α-ergocryptine (CB-154) induced a significant fall in hGH levels in patients with acromegaly, but a significant rise in plasma hGH in normal subjects. 5). The hGH response to L-dopa was compared before and after incomplete removal of pituitary adenoma in 2 patients with active acromegaly. Before surgery L-dopa decreased the plasma hGH levels, while after surgery L-dopa caused a significant increase in plasma hGH levels in both patients. At 10 months after surgery L-dopa decreased the plasma hGH levels again in one patient despite a low basal hGH concentration. These results suggest that the primary disorder of the acromegaly resides in the pituitary level at least in some patients with acromegaly.