Previous studies have shown that functional capillary density (FCD) is substantially reduced by erythrocyte aggregation. However, only supranormal levels of aggregability were studied. To investigate the effect of erythrocyte aggregability at the level seen in healthy humans, the FCD of selected capillary fields in rat spinotrapezius muscle was determined with high speed video microscopy under normal (non-aggregating) conditions and after induction of erythrocyte aggregation with Dextran 500 (200 mg/kg). To examine shear rate dependence, the effect was studied both at normal and reduced arterial pressures (50, and 25 mmHg), the latter achieved by short periods of hemorrhage. In a separate study, volume flow was determined in arterioles (52.1 ± 3.7 µm) under the same conditions. Before Dextran 500 infusion, FCD fell to 91% and 76% of control values, respectively when arterial pressure was reduced to 50 and 25 mmHg. After Dextran 500 infusion, FCD was 96% at normal arterial pressure and fell to 79% and 37% of normal control values at 50 and 25 mmHg. All FCD values were significantly lower after dextran infusion. FCD reduction after lowering arterial pressure or dextran infusion appeared to be due to plasma skimming rather than capillary plugging. Reduction of FCD by dextran at reduced pressure was compensated by increased red blood cell flux in capillaries with red cell flow. We conclude that the level of aggregability seen in healthy humans is an important determinant of FCD only at reduced arterial pressure.