N-[(trans-4-isopropylcyclohexyl)-carbonyl]-D-phenylalanine (A-4166) revealed a new mode of hypoglycemic action with a more rapid onset and a shorter duration of action than the sulfonylureas (SUs). Hypoglycemic mechanisms and glycemic control benefits were demonstrated in laboratory animals. The stimulatory effect of A-4166 on insulin release, in fasting dogs with a cannula into the portal vein, was more rapid than that of tolbutamide after oral administration. A-4166 stopped the stimulation of insulin secretion very quickly, whereas tolbutamide maintained an elevation in plasma insulin levels for at least 6 hours. In the case of A-4166, a counter-regulatory glucagon response was observed during recovery from hypoglycemia, but it was significantly inhibited by tolbutamide. Hyperglycemia induced by glucose loading was rapidly inhibited by A-4166 in normal rats, in genetically diabetic KK mice and in STZ-induced non-insulin-dependent diabetes mellitus (NIDDM) rats. Also, repeated administration of A-4166 for 2 weeks enhanced insulin secretion in the same manner as a single administration in normal rats. In conclusion, A-4166 is a new type of oral hypoglycemic agent, having a rapid and short-term insulin secretory effect and no suppressive effect on the hypoglycemia-induced glucagon response. Oral therapy with A-4166 would be beneficial in supplementing endogenous insulin secretion and would exert ideal glycemic control in NIDDM patients.