In order to better understand the pathophysiologic changes in the immediate postoperative period after total hip replacement surgery and to distinguish alterations due to the surgical operation from those due to bed rest, we examined rheologic parameters and red cell metabolism of patients before, 1 day after, and 5 to 6 days after total hip replacement and compared the results to those obtained from normal volunteers placed at bed rest for 5 days. Bed rest in the control group led to increases in hematocrit, mean corpuscular hemoglobin concentration, red cell DPG and ATP levels, and plasma concentrations of total proteins, globulins, and fibrinogen, with attendant increases in whole blood viscosity, plasma viscosity, blood viscosity, relative blood viscosity with hematocrit adjusted to 45 %, and viscometric aggregation index, and the viscosity of red cell suspensions in Ringer's solution at 45% hematocrit decreased at low shear rate. The patient group, despite the postoperative lowering of their hematocrit, mean corpuscular hemoglobin concentration, and total plasma proteins and a consequent decrease of whole blood viscosity, revealed disproportionate increases in blood viscosity, relative blood viscosity, and viscometric aggregation index. These rheologic changes, which reflect an enhanced red cell aggregability, may contribute to complications of thrombophlebitis. Enforced inactivity, when superimposed on the effects of trauma, blood loss, transfusion with bank blood, and the lowflow state, would exaggerate these rheologic problems. The results suggest that management of total hip replacement patients should include hemorrheologic considerations (e.g., preoperative intentional hemodilution) and early postoperative activity.