In order to investigate whether or not insulin exerts any influence on endogenous insulin secretion, 6 non-obese healthy volunteers were connected to the artificial beta-cell (BIOSTATOR) for 16 hours. After an overnight fast 0.4 U of intermediate acting insulin (Monotard MC/NOVO INDUSTRIES) per kg body weight were injected subcutaneously. The computer program was set to maintain steady-state plasma glucose concentrations at fasting levels by a variable glucose infusion. This approach is able to prevent completely hypoglycemic episodes. Endogenous insulin secretion was evaluated by measuring plasma C-peptide concentrations. Whereas plasma insulin levels increased markedly in response to insulin injection at the same time C-peptide levels declined correspondingly, indicating an inhibition of insulin secretion. Thus, our results provided further support for a negative feedback regulation of insulin secretion by insulin itself.