Abstract The present study was performed to establish whether the mechanism by which the cholinergic system influences growth hormone (GH) release was altered in type I diabetic patients. Therefore, we investigated in a dose-response fashion the inhibitory effect of the muscarinic cholinergic receptor antagonist pirenzepine on the GH responses to arginine infusion (30 g infused intravenously (IV) in 30 minutes), exercise (bicycle ergometer test at an intensity of 75 W for 30 minutes), or 1-44 GH-releasing hormone (GHRH) (1 μg/kg in an IV bolus). In a preliminary study, IV injection of 17.5 mg pirenzepine failed to produce modification in the GH response to arginine infusion in both diabetic (N = 4) and normal subjects (N = 4), and to exercise (diabetics, N = 4; normals, N = 4). Therefore, other subjects were tested without and with 20, 25, and 30 mg pirenzepine during arginine (diabetics, N = 7; normals, N = 7) or exercise (diabetics, N = 7; normals, N = 7) tests. Each subject was tested four times. Diabetic patients presented higher GH responses to stimulation with arginine or exercise than normal controls. In both groups, pirenzepine administered at doses ranging from 20 to 30 mg produced a dose-related inhibition of the GH response to arginine and exercise, which was almost complete when 30 mg pirenzepine was administered. However, the percent inhibition produced by 20 or 25 mg pirenzepine in the arginine test and by 20 mg in the exercise test was significantly lower in the diabetic patients than in the normal controls. Since these results might reflect hyperresponsiveness toward the stimuli for GH release rather than hyporesponsiveness toward the blocking agent, further experiments with GHRH plus pirenzepine (0, 20, 25, or 30 mg) were performed in additional groups of normal (N = 7) and type I diabetic (N = 7) subjects. Results showed similar GH responses to GHRH in controls and diabetics in the absence of pirenzepine. Again, treatment with 20 or 25 mg pirenzepine produced a significantly lower inhibition of the GHRH-induced GH release in diabetics than in normal men, whereas 30 mg pirenzepine almost completely abolished the GH response to GHRH in both groups. These data show a decreased sensitivity to muscarinic cholinergic receptor blockade in the mechanisms underlying the GH responses to arginine, exercise, and GHRH in type I diabetic patients. An increased cholinergic activity at hypothalamic and/or pituitary level might explain this phenomenon and might be, at least in part, responsible for the hyperresponsiveness to arginine and exercise in type I diabetic patients.