Effects of treadmill exercise on osmotic fragility of circulating erythrocytes in jugular venous blood were investigated before and after splenectomy in 3 control and 3 splenectomized thoroughbreds. Both before and after the operation, the fragility of circulating erythrocytes significantly decreased with warming-up, but significantly increased with incremental exercise, and again significantly decreased with cooling down. The fragility of circulating erythrocytes was lower than that of the autologous splenic red cells, and it gradually decreased after the splenectomy. These result suggest that: 1) frequent accumulation in the spleen accelerates the fragility of erythrocytes; and 2) heavy exercise increases fragility, but light exercise decreases it, regardless of the release of red cells from the spleen into the circulation.