Elevation of plasma nonesterified fatty acid (NEFA) levels has been shown in various studies to induce peripheral tissue insulin resistance and impair the suppression of endogenous glucose production (EGP). These studies have been conducted predominantly in men. We compared the effects of elevated plasma NEFA levels on basal and insulin-stimulated glucose metabolism in 8 normal women (age 42 ± 8 years [mean ± SD], BMI 25 ± 3 kg/m²) and 10 normal men (35 ± 6 years, 24 ± 3 kg/m²). Each subject underwent two 5-h 80 mU · m^–2 · min^–1 hyperinsulinemic-euglycemic clamps with measurement of glucose kinetics (intravenous [3-³H]glucose) and substrate oxidation. Plasma NEFA levels were elevated in one study for 3 h before and during the clamp (∼1 mmol/l in both groups) by infusion of 20% Intralipid (60 ml/h) and heparin (900 U/h). In the control studies, the men and women had similar insulin-stimulated glucose disposal rates (R_d) and substrate oxidation rates. In the men, elevated NEFA levels decreased insulin-stimulated glucose R_d during the final 40 min of the clamp by 23% (P < 0.001). By contrast, no significant change in glucose R_d was found in the women (control 10.4 ± 1.1, lipid study 9.9 ± 1.3 mg · kg^–1 · min^–1). Glucose R_d was also unchanged in six women studied at a lower insulin dose (40 mU · m^–2 · min^–1). During the last 40 min of the high–insulin dose clamps with elevated NEFA, glucose oxidation was decreased by 33% in the men (P < 0.001) and by 23% in the women (P < 0.02). Nonoxidative glucose R_d at this time was decreased by 15% in the men (P = 0.02) but was not significantly affected in women. Basal EGP was unaffected by elevation of plasma NEFA levels in both groups. Suppression of EGP during the glucose clamps, however, was impaired. At the insulin infusion rate used, the magnitude of this defect was comparable in men and women. In summary, our findings suggest that although the effects on EGP appear comparable, the inhibitory effects of NEFA on peripheral tissue insulin sensitivity are observed in men but cannot be demonstrated in women.