Insulin and glucagon secretion was compared in women with impaired glucose tolerance (IGT; n = 19, age 58.4 +/- 0.3 yr; mean +/- SD) and women with normal glucose tolerance (NGT; n = 40, age 58.4 +/- 0.3 yr). Fasting plasma insulin levels were higher in IGT than in NGT (P = 0.026), whereas fasting glucose and glucagon levels were not different. Arginine was injected intravenously (5 g), which rapidly stimulated insulin and glucagon secretion in all subjects. Raising the blood glucose (BG) to 14 and 28 mmol/L potentiated insulin secretion and inhibited glucagon secretion. The acute insulin response to arginine (AIR = 2-5 min postload increase) at BG 14 mmol/L, but not at fasting BG or BG 28 mmol/L, was lower in IGT than in NGT (P = 0.033), as was the glucose potentiation of AIR (slopeAIR) (P = 0.020). The acute glucagon response (AGR) was higher in IGT than in NGT at BG 14 mmol/L (P = 0.016). SlopeAGR (glucose inhibition of AGR) was reduced in IGT (P = 0.001). In NGT, there was a significant inverse correlation between slopeAIR and slopeAGR (P = 0.002) not seen in IGT. We conclude that in IGT with normal fasting BG, the glucose modulation of islet function is impaired, indicating that islet dysfunction is an early lesion during the development of noninsulin-dependent diabetes mellitus.