Changes in blood levels of glucagon, insulin and glucose in response to infusions of alanine and glycine have been studied in postabsorptive and fasting obese human subjects. Four-to-five-fold elevations of baseline plasma alanine levels stimulated glucagon secretion significantly. Supraphysiological plasma levels of glycine had a small but insignificant stimulatory effect on glucagon secretion. The glucose increase (6 to 10 mg per 100 ml) observed within 30 min of a supraphysiologic alanine infusion in subjects fasted for 2 or more weeks may be secondary to glucagonmediated glycogen breakdown. However, despite equivalent glucagon rises in the other two study periods, no significant rise in blood sugar was noted during the period of infusion.