Анотация: The aim of the experiment was to study the in vivo effect of exhaustive aerobic exercise on osmotic resistance of erythrocytes, neutrophil/lymphocyte ratio and some clinical parameters in dogs. We used 12 male, mongrel dogs divided into two groups - animals from experimental group were submitted to exhaustive exercise; animals from control group did no exercise. Minimum osmotic resistance, 5 % haemolysis, 50 % haemolysis, 90 % haemolysis and maximum osmotic resistance were measured using an osmotic fragility test in the following dynamics: before exercise (BE), right after exercise (0 h), on 2nd hour, 4th hour, 24th hour, 48th hour, 72nd hour and on 7th and 14th day after exercise. Neutrophil to lymphocyte ratio (N/L), body temperature (BT), heart rate (HR) and breathing rate (BR) were measured in the same dynamics. We found a decrease of red blood cell osmotic resistance in experimental dogs (measured by 50 % haemolysis) on 24th hour, 48th hour and 72nd hour and on 7th and 14th day after exercise, as compared to control group (P<0.05). Surprisingly in the experimental group maximum osmotic resistance (100 % haemolysis) increased on 24th hour after exercise, compared both to initial level and control group (P<0.05). N/L ratio increased significantly in the experimental group on 2nd hour compared to BE level (P<0.05). In conclusion, exhaustive exercise acts as a stressor and affects adversely the fragility of red blood cells