Hemodynamic and oxygen transport were observed in 190 studies before and after administration of 500 ml dextran 40 in 12 normal, healthy subjects and 147 critically ill patients who were, or recently had been, in various degrees of shock. The major influence of dextran 40 was plasma expansion and hemodilution, which resulted in increased blood flow and blood volume. In general, the pressure-flow responses were greater in the ill patients than in the normal subjects. After dextran 40 infusion, oxygen transport increased in the critically ill patients, but not in the normal volunteers. Some of the effects of dextran 40 on oxygen transport in critically ill patients may be attributed to increased tissue perfusion from expansion of plasma volume, as well as to improved flow properties of blood in the microcirculation.