Six subjects performed one-legged dynamic knee-extension. Blood samples were drawn from the femoral artery and vein, and muscle biopsies were obtained from the quadriceps muscle. Leg blood flow was measured by the thermodilution technique, and 3H-inulin was infused for determination of extra- and intracellular muscle water shifts. During the submaximal work load (S) muscle lactate increased, whereas muscle pH remained almost constant; after maximal exercise (M) the values markedly increased for lactate and decreased for pH. Except for a release of lactate from the exercising muscles, K was continuously released throughout S, and this release increased during M. Immediately when the muscles relaxed, the K release was converted to a K re-uptake. The calculated K loss, based on v- a and flow values, agreed with the decrease in muscle K content from 458 mmol/kg dw at rest to 414 mmol/kg dw at exhaustion (P less than 0.05), as analyzed on the muscle biopsies. Muscle water content increased during S mainly because of an increased extracellular H2O, whereas during M the largest increase occurred in intracellular H2O (H2Oi). Because of the simultaneous K loss and H2Oi increase in the exercising muscle the intracellular [K] was calculated to decrease from 165 mM at rest to 129 mM at exhaustion. This decrease and an increase in extracellular [K] from 4.5 mM at rest to greater than 6.0 mM at exhaustion affects the muscle membrane excitability. Muscle fatigue may thus not only be caused by changes within the cell, affecting energy metabolism or actin-myosin reaction, but may be located at the membrane protecting the cell against overload.