ABSTRACT We have reported inhibitory effects of hyperglycemia upon l-dopa-induced growth hormone (GH) stimulation in normal subjects and in hyperglycemic patients due to diabetes mellitus. Ten normal subjects whose GH rose greater than 10 ng/ml following 1.0 g of l-dopa orally were given an iv glucose injection and a continuous infusion to maintain hyperglycemia. Hyperglycemia prevented the rise of GH following administration of the same dose of l-dopa. Five diabetic subjects who were hyperglycemic also failed to respond to l-dopa. These results show that hyperglycemia prevents l-dopa-induced GH stimulation.