Glucose induces insulin secretion and also potentiates the insulin-releasing action of secretagogues such as arginine and sulfonylureas. This potentiating effect is known to be impaired in type-2 diabetic patients, but its cellular mechanisms are unclear. Insulin secretion (IS) and cytosolic Ca²⁺ concentration ([Ca²⁺]_i) were measured in mouse islets during perifusion with 3-15 mmol/l glucose (G), and pulse or stepwise stimulation with 1-10 mmol/l arginine or 5-250 µmol/l tolbutamide. In G3, arginine induced small increases in [Ca²⁺]_i but no IS. G7 alone only slightly increased [Ca²⁺]_i and IS, but markedly potentiated arginine effects on [Ca²⁺]_i, which resulted in significant IS (already at 1 mmol/l). For each arginine concentration both responses further increased at G10 and G15, but the relative change was distinctly larger for IS than [Ca²⁺]_i. At all glucose concentrations, tolbutamide dose-dependently increased [Ca²⁺]_i and IS with thresholds of 25 µmol/l for [Ca²⁺]_i and 100 µmol/l for IS at G3, and of 5 µmol/l for both at G7 and above. Between G7 and G15, the effect of tolbutamide on [Ca²⁺]_i increased only slightly whereas that on IS was strongly potentiated. The linear relationship between IS and [Ca²⁺]_i at increasing arginine or tolbutamide concentrations became steeper as the glucose concentration was raised. Thus, glucose augmented more the effect of each agent on IS than that on [Ca²⁺]_i. In conclusion, glucose potentiation of arginine- or tolbutamide-induced IS involves increases in both the rise of [Ca²⁺]_i and the action of Ca²⁺ on exocytosis. This dual mechanism must be borne in mind to interpret the alterations of the potentiating action of glucose in type-2 diabetic patients.