The ion exchanger K microelectrode was used to assess the K concentration in the interstitial fluid of [rat] skeletal muscle in the control and shock states. Concomitant resting membrane potential measurements and the analysis of fluid and electrolyte content of biopsies of the muscle were used to detect defects in cellular function. The changes occurring in hemorrhagic shock were consistent with a depression of resting transmembrane potential, depletion of extracellular fluid and cellular swelling with Na uptake and K loss. The disproportional increase of interstitial K concentration over that of the plasma in low flow states appeared to be a result of inhibited Na-K transport in cell membranes concomitant with an equilibration delay between these 2 extracellular compartments. The significant increase in K concentration in the interstitium may have potentiated other changes in cardiovascular and respiratory functions which have been observed in the complex picture of hemorrhagic shock.