Background: A contracting muscle is a source of the plasma K+ concentration increase during physical exercise. The flux of K+ from contracting skeletal muscle to blood is related to the frequency of cells action potential. The elevated blood [K+] may result in the heart rate irregularities and interferes with the way nerves send signals. But plasma increased [K+] recovers rapidly to normal if a regulating mechanism takes action. The aim of this study was to evaluate the participation of processes restoring the balance in blood [K+] after prolonged submaximal exercise. Material/Methods: Nineteen healthy, young, physically active men performed the 120-min submaximal cycling (intensity below individual AT). Measurements were made of urine, plasma and hemolysed whole blood collected before and after a 2-h cycloergometric exercise and after 1h, 2h and also after 24h recovery to quantify the excretion of K+ to urine and the relative contribution of plasma and erythrocytes to the place where K+ is released in two compartments of blood. Results: The main findings in the present study are that the balance of plasma [K+] after prolonged exercise is maintained not only by the reuptake of K+ to a non-contracting and contracting muscle and by changes in [K+] in erythrocyte and plasma, but as well by the excretion of ions into urine. Conclusions: The fate of K+ released from a contracted muscle is connected not only with the exercise intensity and acidification but also with the duration of exercise. Athletes should keep in mind different action of kidneys in case of K+ before supplementation of electrolytes after specific exercise.