The effect of exercise on the intraerythrocyte cationic concentrations and transmembrane fluxes such as the Na+-K+-adenosinetriphosphatase (ATPase) pump, the Na+-K+ cotransport, and the Na+-Li+ countertransport system was studied in 11 normal male volunteers. All subjects performed an uninterrupted incremental exercise test on a bicycle ergometer, starting at an initial work load of 20% of the subjects' maximal exercise capacity, as determined in a pretest. The work rate was increased with an additional 20% each 6 min up to a final work load of 80%. Blood samples were taken at rest, at 60 and 80% of maximal exercise capacity, and 1, 2, 3, 4, 5, and 30 min after cessation of exercise. At moderate exercise (60% of maximal exercise capacity) the intraerythrocyte potassium concentration was not changed, but at severe exercise (80% of maximal exercise capacity) it was decreased. After exercise the intraerythrocyte potassium concentration returned to base line within 2 min. Exercise did not affect the intraerythrocyte concentrations of sodium and magnesium. The activity of the Na+-K+-ATPase pump and the Na+-K+ cotransport in the erythrocytes during and after exercise was no different from the resting level. The activity of the Na+-Li+ countertransport system on the contrary tended to decrease during exercise. It is concluded that exercise is accompanied by a leakage of potassium out of the erythrocytes without major alterations in the active red cell cationic fluxes.