The secretion of GH in two siblings with clinical dwarfism and high GH plasma levels (the mean of several basal values; 233.83 ng/ml in patient A and 178.16 in patient B has been studied with several dynamic tests. An arginine infusion increased GH levels in both cases (+193.55% for A, +140.27% for B). No significant modifications were obtained with oral glucose tolerance test +18.70% for A, +24.32% for B). A bolus of somatostatin almost completely prevented the rise in GH levels in response to arginine. Pretreatment with bromocryptine clearly increased basal GH plasma levels (A, +58.66%; B, +56.03%) and the response to arginine. As in the case of a normal hypothalamus, the hypothalamus of Laron's syndrome responds to arginine and bromocryptine, with GH elevations. Somatostatin suppresses GH levels. A lack of response to glucose can be considered as a nonspecific effect of the very low biological activity of the stimulus in a hyperstimulated hypothalamus. We suggest that GH secretion by the hypothalmo pituitary system in Laron's syndrome is normal, and that GH hyperproduction may be due to a generalized defect in GH receptors or to the low levels of somatomedin.