At 78 and 110 F hepatic clearance of indocyanine green (ICG), O2 intake, heart rate, blood lactate, and rectal temperature were measure on nine men unacclimatized to heat during treadmill exercise, requiring 45–95% of maximal O2 intake (max Vo2). Percentage of resting ICG clearance was inversely proportional to percentage of maximal O2 intake at 78 F (ggr = -0.78) and 110 F (ggr = -0.81). Clearance of ICG was 20% less at 110 F than at 78 F at all metabolic rates above 26% of maximal Vo2. Measurements of hepatic blood flow in three men at 110 F validate these estimates of percentage decrements in hepatic blood flow. Submaximal and maximal Vo2 and maximal heart rates were unaltered by heat, but maximal heart rates were reached during submaximal work at 110 F. Decreased work capacity at 110 F was unrelated to rectal temperature or blood lactate; the latter was unaffected by temperature. The liver and the kidneys may divert to the skin sufficient blood to obviate the need for additional increments in cardiac output during work at high temperature. Maximal decrements in hepatic blood flow at lower work intensities may contribute to diminished work capacity at 110 F. hepatic blood flow during exercise; lactic acid and heat stress; rectal temperature; heart rate and temperature; maximal o2 intake, influence of temperature; temperature redistribution of blood flow; work capacity, effect of heat Submitted on October 30, 1964

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