The effect of long-term tourniquet ischemia on electrolyte levels in muscle tissue and plasma during and after occlusion was investigated in man. Muscle tissue electrolytes were studied in nine patients using a percutaneous muscle biopsy technique. The estimation of the intra- and extracellular electrolyte content was based on the determination of the chloride space. In another similar group of patients (n=11) plasma electrolytes were investigated by taking blood samples from fine plastic catheters inserted into both femoral veins and one radial artery. Capillary blood flow was measured using the 133Xenon-clearance technique. During occlusion there were no significant changes in muscle tissue or plasma electrolytes. After release of the tourniquet an increase in plasma potassium of the operated leg and arterial plasma were observed. Maximal muscle blood flow and maximal potassium values occurred at the same time giving a pronounced potassium release from the operated leg. The osmol release from the occluded leg showed a maximum 1 min after tourniquet release. The total and intracellular content of potassium and sodium in the muscle tissue did not significantly alter after release of the occlusion. A moderate increase in extracellular water was found after tourniquet release.