Abstract. In order to assess whether patients with noninsulin-dependent diabetes mellitus (NIDDM) possess normal insulin secretory capacity, maximal B cell responsiveness to the potentiating effects of glucose was estimated in eight untreated patients with NIDDM and in eight nondiabetic controls. The acute insulin response to 5 g intravenous arginine was measured at five matched plasma glucose levels that ranged from 100-615 mg/dl. The upper asymptote approached by acute insulin responses (AIRma.) and the plasma glucose concentration at half-maximal responsiveness (PG50) were estimated using nonlinear regression to fit a modification of the Michaelis-Menten equation. In addition, glucagon responses to arginine were measured at these same glucose levels to compare maximal A cell suppression by hyperglycemia in diabetics and controls.Insulin responses to arginine were lower in diabetics than in controls at all matched glucose levels (P < 0.001 at all levels). In addition, estimated AIRma. was much lower in diabetics than in controls (83±21 vs. 450±93 MU/ml, P < 0.01). In contrast, PG50 was similar in diabetics and controls (234±28 vs. 197±20 mg/dl, P equals NS) and insulin responses in both groups approached or attained maxima at a glucose level of -460 mg/dl. Acute glucagon responses to arginine in patients with NIDDM were significantly higher than responses in controls at all glucose levels. In addition, although glucagon responses in control subjects reached a minimum at a glucose level of -460 mg/dl, responses