Using a hematocrit monitoring system and a Swan-Ganz catheter, the authors investigated the changes in blood volume and hemodynamic parameters before, during, and after dialysis-induced hypotension. When hypotension occurred, the cardiac output dropped sharply, and there were concomitant sudden decreases in the mean pulmonary arterial pressure and the mean right atrial pressure. These are attributed to a decrease in venous return. Since there was no recognizable change in blood volume when the blood pressure fell sharply, decrease in the venous return is considered to be caused by relocation of circulating blood, possibly associated with a sudden decrease in venous tone.