In eight normal weight healthy volunteers it was proved by C-peptide assay whether high concentrations of insulin in serum would suppress the secretion of insulin in an autoregulative manner. Insulin combined with glucose, therefore was given intravenously as a single bolus injection as well as an infusion over a period of two hours. Moreover it was proved whether high levels of insulin would suppress the reactivity of the B-cell to sulphonylurea administration. It was demonstrated that the secretion of the B-cell is regulated only by the concentration of the blood sugar, but not by the level of serum insulin. Likewise the stimulation of the B-cell by sulphonyl-urea administration is not suppressed by high concentrations of insulin.