Knowledge about changes in the osmotic fragility of erythrocytes during exercise in athletic horses which is introduced in this review can be summarized as follows; 1) Changes in the osmotic fragility of equine erythrocytes during exercise are an important indicator of intravascular haemolysis. 2) The sensitivity of the regulatory volume is decreased by a K-Cl cotransporter, lactate and/or pH susceptibility, and the osmotic fragility of erythroid membrane due to exercise stress increases with the repeated accumulation of red cells in the spleen. 3) The increase in blood flow and the release of erythrocytes from the spleen into the circulation have little effect on the changes in the osmotic fragility of erythrocytes during exercise. 4) Increases in blood pH and temperature caused by aerobic exercises cause osmotic resistance in erythrocytes. 5) The decrease in pH and increase in blood lactate and peroxide caused by anaerobic exercises promotes the osmotic fragility of erythrocytes. 6) The osmotic fragility of erythrocytes could be detected by the modification, degeneration, decomposition and oxidation-reduction in cell membrane lipids and proteins, and cell volume regulation, changes which follow pH and temperature changes during exercise. 7) The K-Cl cotransporter which controls the intracellular ionic homeostasis and regulatory volume decrease has a functional interrelation with the fragility of erythrocytes. The osmotic fragility of erythrocytes in athletic horses is therefore an indicator of the effects of training and/or athletic performance and stress.