In situations where capillary perfusion in skeletal muscles is limited, changes in blood flow play an important role. Especially alterations in intrinsic erythrocyte factors like red cell aggregation and deformability would increase blood flow resistance. In our study we investigated whether whole blood and plasma viscosity influence exercise tolerance during submaximal training and whether a difference can be realized between trained and untrained standardbred trotters. Venous blood from 42 healthy adult trotters (20 horses at the beginning of their training and 22 well trained horses) was investigated before, immediately afterwards and 30 minutes after submaximal exercise. In both groups whole blood viscosity (WBV; LS30, Contraves, Switzerland) increased significantly (p<0.001) at all shear rates (94 s-1, 2.4 s-1, 0.7 s-1) and decreased after 30 minutes to baseline, as did plasma viscosity (PV; OCR-D, Paar, Austria; p<0.001). No changes in WBV could be seen in hematocrit (40%) standardized samples. Erythrocyte aggregation (EA) indices increased (Myrenne, Germany; p<0.001). Creatin-kinase (p<0.001), lactate (p<0.001), hemoglobine (p<0.001), heart rate and oxygen saturation (p<0.05) increased, while PH (p<0.05), and BE (p<0.001) decreased during the race. In our study, submaximal exercise was related to remarkable changes in hemorheologic variables in the single animal. It was also shown that exercise resulted in a more extensive change of fluidity in well trained horses in comparison to untrained animals, at least for low shear WBV and EA (p<0.05).