Concentration of 2,3-DPG was determined in erythrocytes of 30 young cyclists at rest, immediately after a strenuous physical exercise on a bicyle- ergometer, and then in 30th min rest following the exercise. It was found that exercise changes the concentration of 2,3-DPG in the erythrocytes and the trends of these changes were different depending on endurance of the subjects examined. In the subjects with higher endurance a tendency was observed for decreasing concentration of 2,3-DPG immendiately after the exercise with a futher significant fall after a 30 min rest. On the other hand, in the subjects with lower endurance the erythrocyte 2,3-DPG concentration rose after the exercise and returned to its initial value at 30 min of rest.