To determine the direct effects of hematocrit on intestinal oxygen consumption and to determine whether an optimal hematocrit exists for intestinal oxygenation, we perfused isolated canine gut loops at a constant pressure (120 mm Hg) and varied hematocrit from 80% to 10%. As hematocrit fell, blood flow rose while arterial oxygen content fell. The regression of blood flow on hematocrit was linear, whereas the relationship between oxygen uptake and hematocrit was parabolic, showing a maximal oxygen uptake at an hematocrit of 48.7%. To determine whether the optimal hematocrit for intestinal oxygenation could be altered by changes in vasomotor tone, we performed two other series of experiments. Raising perfusion pressure to 180 mm Hg did not significantly alter the optimal hematocrit for oxygen uptake. However, when we increased the oxygen demands of the gut by placing transportable solutes within the intestinal lumen, the optimal hematocrit for oxygen uptake increased markedly. We conclude that the optimal hematocrit for intestinal oxygenation is slightly higher than the normal range, a finding that could possibly be explained by the plasma skimming known to occur in the intestinal mucosa. Our experimental design and method of data analysis could be used to determine the optimal hematocrit in other organs.