The role of the exercise-induced fall in insulin in fat metabolism was studied in dogs during 150 min of treadmill exercise alone (controls) or with insulin clamped at basal levels by an intraportal infusion to prevent the normal fall in insulin concentration (ICs). To counteract the suppressive effect of insulin on glucagon release, glucagon was supplemented by an intraportal infusion in ICs. In all dogs, catheters were placed in a carotid artery and in the portal and hepatic veins for sampling and in the vena cava and the splenic vein for infusion purposes. Glucose levels were clamped in ICs to recreate the glycemic response evident in controls. In controls, insulin fell by 7 ± 1 μxU/ml but was unchanged from basal levels in ICs (0 ± 2 μU/ml). Glucagon, norepinephrine, epinephrine, and cortisol rose similarly in controls and ICs. Arterial free-fatty acid (FFA) levels rose by 644 ± 126 μeq/L in controls but did not increase in ICs (−12 ± 148 μeq/L). Arterial glycerol levels rose by 337 ± 43 and 183 ± 19 (μM in controls and ICs. Hepatic FFA delivery and fractional extraction increased by 17 ± 3 and 0.06 ± 0.02 μmol · kg⁻¹ · min⁻¹, respectively, in controls. In ICs, hepatic FFA delivery increased by only 1 ± 2 (μmol · kg⁻¹ · min⁻¹, whereas hepatic fractional extraction fell slightly (−0.03 ± 0.03). Consequently, net hepatic FFA uptake rose by 4.8 ± 1.5 μxmol · kg⁻¹ · min⁻¹ in controls but decreased slightly in ICs (−0.5 ± 1.1 μmol · kg⁻¹ · min⁻¹). At least partly because of the differences in hepatic FFA uptake, arterial β-hydroxybutyrate (50 ± 14 vs. 23 ± 8 μM) and net hepatic β-hydroxybutyrate output (2.2 ± 0.7 vs. 0.4 ± 0.3 μmol · kg⁻¹ · min⁻¹) rose more in controls than in ICs. In summary, the exercise-induced fall in insulin 7) is essential to the increase in FFA levels during prolonged muscular work, 2) facilitates hepatic FFA uptake by enhancing the delivery of FFAs to the liver and their extraction by the liver, and 3) enhances the net β-hydroxybutyrate output by the liver, at least in part through the effects described in 1 and 2. Hence, the exercise-induced fall in insulin is essential for the transition to the increased rate of FFA metabolism that occurs as work duration progresses.