Five healthy males performed four 30-s bouts of maximal isokinetic cycling with 4 min rest between each bout. Arterial and femoral venous blood was sampled during and for 90 min following exercise. During exercise, arterial erythrocyte [K+] increased from 117.0 ± 6.6 mequiv./L at rest to 124.2 ± 5.9 mequiv./L after the second exercise bout. Arterial erythrocyte [K+] returned to the resting values during the first 5 min of recovery. No significant change was observed in femoral venous erythrocyte [K+]. Arterial erythrocyte lactate concentration ([Lac−]) increased during exercise from 0.2 ± 0.1 mequiv./L peaking at 9.5 ± 1.5 mequiv./L at 5 min of recovery, after which the values returned to control. Femoral venous erythrocyte [Lac−] changed in a similar fashion. Arterial erythrocyte [Cl−] rose during exercise to 76 ± 3 mequiv./L and returned to resting values (70 ± 2 mequiv./L) by 25 min recovery. During exercise there was a net flux of Cl− into the erythrocyte. We conclude that erythrocytes are a sink for K+ ions leaving working muscles. Furthermore, erythrocytes function to transport Lac− from working muscle and reduce plasma acidosis by uptake of Cl−. The erythrocyte uptake of K+, Lac−, and Cl− helps to maintain a concentration difference between plasma and muscle, facilitating diffusion of Lac− and K+ from the interstitial space into femoral venous plasma.Key words: isokinetic cycle ergometer, ion movement, hydrogen ion, cell volume, arterial blood.