Effects of echinocytosis on blood rheology and exercise performance were evaluated for 5 Thoroughbreds. Echinocytosis was induced by administration of furosemide (1 mg/kg of body weight, IM, q 12 h) for 4 days. Furosemide treatment resulted in decreases in serum sodium and serum chloride concentrations and in RBC chloride and potassium concentrations. Echinocytosis was associated with increased RBC density as determined by RBC density gradient centrifugation. However, samples containing echinocytes were more filterable than control samples, indicating that echinocytes were not rigid cells. Erythrocyte sedimentation rate was decreased in blood samples containing echinocytes, indicating that cell-to-cell interaction was reduced. Whole blood viscosity was not altered by presence of echinocytes. Echinocytes did not impair the capacity of horses to complete treadmill exercise tests, nor did they alter heart rate or blood gas variables. However, plasma lactate concentration was higher in samples obtained during exercise at a treadmill speed of 9 m/s. Echinocytosis was associated with higher postrace creatine kinase activity. These data indicate that echinocytes may be dense, but not rigid cells, which have decreased tendency to aggregate and do not increase whole blood viscosity. Therefore, echinocytes are unlikely to inhibit or obstruct microvascular blood flow.