In a previous report, butylbiguanide was found to increase the oral glucose tolerance in normal human subjects, but left the K-value unchanged during rapid intravenous tolerance tests. The present report shows, that butylbiguanide increases the glucose tolerance of normal human subjects even when glucose is given intravenously over a long period of time (6-hour infusion). The radioimmunological serum insulin (lRI) was not significantly changed. The free fatty acids (FF A), however, were significantly lowered. This appeared to be a direct effect of the drug which could account for its blood glucose lowering action.