The effect of alpha and beta adrenergic blocking agents on the basal serum levels of glucose, NEFA and insulin was studied in normal patients. It was shown that propranolol (β-blocking agent) produced a significant decrease in each of these variables. On the other hand, the opposite effect was obtained, except for glucose, when an α-blocking agent, phentolamine, was used. These findings suggest that circulating catecholamines may regulate the above-mentioned parameters through an equilibrium between two antagonistic effects.