Abstract We investigated the influence of an insulin-induced hypoglycemia on plasma glucagon in nonpregnant healthy young women and in women during the last month of gestation. Both groups were tested either in the basal state or during a period where free fatty acid plasma levels were increased by infusion of a lipid emulsion supplemented with heparin. Regular insulin was injected intravenously at the dose of 0.1 U/kg body wt in controls and 0.3 U/kg in pregnant women in order to obtain a similar lowering of blood glucose in all groups. In controls, the increase in plasma glucagon was maximum 30 and 45 min after insulin injection and averaged 130 pg/ml; the infusion of triglycerides and heparin which raised plasma FFA to about 1300 μEq/liter decreased basal plasma glucagon levels and reduced, by about 70%, the glucagon response to hypoglycemia. During the last month of pregnancy, the glucagon response to insulin-induced hypoglycemia was reduced by 60% (mean maximal increase 52 pg/ml); furthermore, raising plasma FFA to about 1500 μEq/liter completely abolished the glucagon rise induced by the insulin hypoglycemia. These results support the view that the glucagon release from A-cells can be modulated by the level of circulating plasma FFA.