We tested whether FFAs influence glucose uptake by human peripheral tissues in vivo. Whole body glucose uptake, FFA turnover, energy expenditure and substrate oxidation rates, forearm glucose and FFA uptake, and nonoxidative glycolysis (net release of alanine and lactate) were measured in 14 normal male subjects in the basal state (0–240 min; serum insulin, ∼5 μU/mL) and during euglycemic hyperinsulinemia (240–360 min; ∼75 μU/mL) on 2 separate occasions, once during elevation of plasma FFA by infusions of Intralipid and heparin (plasma FFA, 4.6 ± 0.1 vs. 4.2 ± 0.4 mmol/L; 180–240 vs. 300–360 min) and once during infusion of saline (plasma FFA, 0.50 ± 0.07 vs. 0.02 ± 0.07 mmol/L, respectively). In the basal state, whole body glucose disposal remained unchanged, but the fate of glucose was significantly altered toward diminished oxidation (7.3 ± 0.8 vs. 5.6 ± 0.5 μmol/kg·min; P < 0.05, saline vs. Intralipid) and increased nonoxidative glycolysis (P < 0.05). Elevation of plasma FFA significantly increased forearm glucose uptake (1.0 ± 0.6 vs. 2.4 ± 0.7 μmol/kg-min; P < 0.01) and nonoxidative glycolysis (net release of alanine and lactate, 0.4 ± 0.5 vs. 1.2 ± 0.4 μmol glucose equivalents/kg·min; P < 0.05). During hyperinsulinemia, FFA decreased whole body glucose disposal (38 ± 2 vs. 30 ± 3 μmol/kg·min; P < 0.001) due to a decrease in glucose oxidation (13 ± 1 vs. 7 ± 1 μmol/kg·min; P < 0.01, saline vs. Intralipid), and forearm glucose uptake (31 ± 4 vs. 24 ± 6 μmol/kg·min; P < 0.01, saline us. Intralipid). Under these conditions, 7 ± 2% and 3±1% (P < 0.05) of forearm glucose uptake could be accounted for by nonoxidative glycolysis in the Intralipid and saline studies, respectively. In summary, 1) elevation of plasma FFA concentrations suppresses the rate of carbohydrate oxidation to a rate that, both basally and during hyperinsulinemia, is similar to that reported for insulin-independent glucose oxidation in the brain; 2) basally, forearm glucose uptake is increased by FFA; and 3) during hyperinsulinemia, FFA inhibit glucose uptake by forearm tissues. We conclude that the interaction between glucose and FFA fuels in human forearm tissues is dependent upon the ambient insulin concentration; the increase in basal glucose uptake would be compatible with the increased need of glucose for FFA reesterification; the decrease in insulin-stimulated glucose uptake supports operation of the glucose-FFA cycle in human forearm tissues.