There is strong support for the notion that free fatty acids (FFAs) are an important link between obesity, insulin resistance, and type 2 diabetes. The evidence can be summarized as [...], To study effects of sex on free fatty acid (FFA)-induced insulin resistance, we have examined the effects of acute elevations of plasma FFA levels on insulin-stimulated total body glucose uptake in nine healthy young women. Euglycemic-hyperinsulinemic (~500 pmol/l clamps were performed for 4 h with coinfusion of either lipid/heparin (L/H) to acutely raise plasma FFA levels (from ~600 to ~1,200 µmol/l) or saline/glycerol to lower fatty acids (from ~600 to ~50 µmol/l). L/H infusion inhibited insulin-stimulated glucose uptake (determined with [3-³H]glucose) and glycogen synthesis by 31 and 40%, respectively (P < 0.01), almost completely abolished insulin suppression of endogenous glucose production (EGP) (13.6 vs. 10.0 µmol - [kg.sup.-1] * [min.sup.-1], NS), prevented the insulin induced increase in carbohydrate oxidation (8.1 vs. 7.4 µmol * [kg.sup.-1] * [min.sup.-1], NS), and stimulated fat oxidation (from 3.6 to 5.1 µmol * [kg.sup.-1] * [min.sup.-1], P < 0.01). These data showed that acute increases in plasma FFA levels inhibited the actions of insulin on glucose uptake, glycogen synthesis, and EGP in women to a degree similar to that previously reported in men. We conclude that at insulin and FFA levels in the postprandial range, women and men were susceptible to FFA-induced peripheral and hepatic insulin resistance.