Obesity is associated with an impairment of the GH secretion elicited by all stimuli known to date, but the basic mechanism of this alteration is unknown. To determine whether obesity is associated with a chronic state of tonic somatostatin secretion, several tests with GH stimuli with or without pyridostigmine were undertaken in both obese subjects and matched controls. Pyridostigmine reduces somatostatin release from the hypothalamus by increasing central cholinergic neurotransmission. The administration of clonidine (300 micrograms, orally) to obese subjects did not modify basal GH values (1.9 +/- 0.7 micrograms/L at 90 min), while in control subjects the clonidine-induced GH peak was 13.1 +/- 1.6 micrograms/L. Pretreatment with pyridostigmine (120 mg, orally) notably increased clonidine-stimulated GH secretion in both the obese (6.9 +/- 1.8 micrograms/L) and control (17.6 +/- 2.7 micrograms/L) subjects. Since clonidine acts by releasing endogenous GHRH, similar studies were undertaken employing arginine, which presumably enhances GH release by reducing somatostatin discharge. Arginine administration in obese subjects induced an increase in GH levels of 5 +/- 2.3 micrograms/L, which was significantly smaller than that in the matched control subjects (13.3 +/- 2.4 micrograms/L). Pretreatment with pyridostigmine increased the arginine action toward a GH peak of 12.2 +/- 2.2 micrograms/L in the obese and 21.6 +/- 2.5 micrograms/L in control subjects. As a third hypothalamic stimulus of GH secretion, trials of insulin-induced hypoglycemia were carried out. Hypoglycemia induced an increase in GH levels in obese subjects of 12.2 +/- 1.8 micrograms/L, which was higher than that produced by any other stimulus, but lower than that in control subjects (28.4 +/- 5.5 micrograms/L). In contrast with the previous two GH stimuli, pretreatment with pyridostigmine did not modify the hypoglycemia-induced GH release in either obese or normal subjects. Our results lend support to the view that clonidine acts through GH-releasing hormone release and arginine by reducing somatostatin discharge from the hypothalamus. In addition, they seem to indicate that hypoglycemia acts by a combination of both mechanisms, mainly through a reduction in somatostatin release. These findings support the idea that obesity is associated with a state of chronic somatostatin hypersecretion as the basis for the derangements in GH secretion.