Prolonged infusions of lipid and heparin which achieve high physiologic FFA concentrations inhibit hepatic (and peripheral) insulin sensitivity in man. These infusions are composed largely of polyunsaturated fatty acids (linoleic and linolenic). It is not known if fatty acid composition per se affects hepatic glucose metabolism in man. To address this issue, we examined the impact of enteral infusions of either palm oil (48% palmitic acid, 35% oleic and 8% linoleic) or safflower oil (6% palmitic, 12% oleic, 74% linoleic) in fourteen obese non-diabetic subjects. ²H₂O was administered to determine the contribution of gluconeogenesis to EGP and a primed continuous infusion of 6,6-d-glucose was administered to assess glucose appearance. As a result of the lipid infusions, plasma FFA concentrations increased significantly in both the palm oil (507.5±47.4 to 939.3±61.3 µmol/L, p< 0.01) and safflower oil (588.2.0±43.0 to 857.8±68.7 µmol/L, p< 0.01) groups after 4 hours. Endogenous glucose production (EGP) was similar at baseline (12.4±1.8 vs 11.2±1.0 µmol.kg FFM^-1.min^-1). During a somatostatin-insulin clamp, the glucose infusion rate was significantly lower (AUC glucose infusion rate 195.8±50.7 vs 377.8±38.0 µmol/kg FFM , p<0.01) and rates of EGP were significantly higher (10.7±1.4 vs 6.5±1.5 µmol.kg FFM^-1.min^-1, p<0.01) after palm oil compared to safflower oil, respectively. Baseline rates of gluconeogenesis and glycogenolysis were also similar. However, following lipid infusion, rates of glycogenolysis were suppressed by safflower oil but not by palm oil. Thus these studies demonstrate, for the first time in man, a differential effect of SFA and PUFA on hepatic glucose metabolism.