To assess the effects of angiotensin converting enzyme (ACE) inhibitor on insulin action in obesity, five normotensive non-diabetic obese women were examined on two occasions as part of a double-blind, randomized, cross-over study involving ten days of treatment with either 1.25 mg ramipril or placebo. The study consisted of a euglycaemic hyperinsulinaemic clamp (two periods of insulin infusion at rates of 0.4 and 1 mU/kg/min, 2 h for each step) combined with indirect calorimetry. The most notable results involved a significantly faster time-course of glucose infusion rates during the first 30 min of each insulin infusion period [analysed by calculating slopes (S1 and S2)] after ramipril than placebo administration. The mean glucose infusion rates reached during the last 30 min of each insulin infusion period (G1 and G2), as well as the increases in carbohydrate oxidation rates during the clamp (C1-C0 and C2-C0) and the decreases in plasma nonesterified fatty acids (A0-A1 and A0-A2), were not significantly different after ramipril and placebo. According to robust principal component analysis of S1, S2, G1, G2, C1, C2, A1 and A2 (orthogonally to C0 and A0), insulin sensitivity was improved with ramipril as compared to placebo (p = 0.013). This study strongly suggests that a low dose of an ACE inhibitor increases the activation phase of insulin action in normotensive nondiabetic obese patients and may accelerate insulin action.