The purpose of the present study was to obtain information on erythrocyte aggregate formation in vivo. The movements of erythrocytes in postcapillary venules of the rat spinotrapezius muscle at various flow rates were recorded with a high-speed video camera before and after infusion of dextran 500. To distinguish aggregates, the following criteria were used: 1) a fixed distance (4 m) between the center points of two adjacent cells, 2) lack of visible separation between the adjacent cells, and 3) movement of the adjacent cells in the same direction. Without dextran 500 infusion, 11 and 5% of erythrocytes formed aggregates in low (33.2 Ϯ 28.3 s) and high pseudoshear (144.2 Ϯ 58.3 s) conditions, respectively, based on the above criteria. After dextran 500 infusion, 53% of erythrocytes satisfied the criteria in the low pseudoshear condition (26.5 Ϯ 17.0 s) and 13% of erythrocytes met the criteria in the high pseudoshear condition (240.0 Ϯ 85.9 s), indicating erythrocyte aggregation is strongly associated with shear rate. Approximately 90% of aggregate formation occurred in a short time period (0.15-0.30 s after entering the venule) in a region 15 to 30 m from the entrance. The time delay may reflect rheological entrance conditions in the venule.hemodynamics; in vivo blood rheology; in vivo microscopy RED BLOOD CELL AGGREGATION is a prominent feature in humans and other athletic species but not sedentary species (30). Numerous in vitro studies have shown that aggregation of blood increases as shear rate decreases. Aggregation also depends on hematocrit and the concentration of macromolecules in the plasma or suspending medium (7,10,15,16,19,25,32). However, the circumstances in which aggregation occurs in vivo and its possible importance in circulatory function are not well understood.Previous whole organ studies in the dog and cat, in which red blood cell aggregation normally occurs, have shown that venous vascular resistance in skeletal muscle increases as arterial pressure and blood flow rate decrease and provided evidence that this effect is dependent on red blood cell aggregation (11,23,24,35). From these studies, it was postulated that aggregate formation increased resistance to flow in the venular network. Recently, Bishop and co-workers (9) reported that, in rodents treated with high molecular mass dextran to induce aggregation, velocity profiles of blood flow in venules became blunted at low flow rates and red blood cells formed aggregates, which would explain, in part, the dependence of venous resistance on flow rate.