1. The rise of plasma [K+] during high intensity exercise is due to an initially rapid loss of K+ from the exercising muscle to the circulation. The K+ loss is primarily governed by the balance between K+ efflux rate from the muscle cells and the reuptake rate. It has been assumed that the reuptake rate is proportional to [K+] in the femoral vein ([K+]fv) during short-lasting uphill running, but this may not hold true for other types and durations of exercise. 2. In four subjects, initial rates of increase and decay of [K+]fv at start and end of bicycle exercise were quantified by means of K(+)-sensitive electrodes inserted into the femoral vein. Responses to exercise intensities between 90 and 440 W were examined. Both the initial rate of rise and the rate of decay of [K+]fv were linearly related to power. 3. In six subjects, exercising at 60, 85 and 110% of maximal oxygen uptake, blood was obtained from the femoral artery and vein. The veno-arterial concentration difference for K+ across the exercising leg decayed with half-times of about 3 min at all exercise levels and became not significantly different from zero at low powers. This fits with a good match between K+ efflux and reuptake rates at the cellular level. 4. Arterial plasma [K+] ([K+]a) rose faster with increasing exercise intensity, reaching peak values of 5.7 +/- 0.1, 6.0 +/- 0.2 and 8.0 +/- 0.2 mmol l-1. [K+]a fell again over the subsequent 5 min at the lowest intensity in spite of significant loss of muscle K+. Hence, released K+ was redistributed to other compartments outside the vascular bed. 5. While K+ loss increased linearly with increasing power, [K+]a showed a curvilinear relationship. Thus redistribution of K+ is less efficient at high intensities. [K+]a correlated better with relative work load than with absolute work load. 6. Reuptake of K+ after the end of the high intensity bout of exercise caused [K+]a to fall with a half-time of 31 s. The rate of K+ reuptake in the exercising muscle was not proportional to [K+]a or [K+]fv. However, at the level of the muscle cell, the rate of K+ reuptake was probably inversely related to intracellular [K+].