K+-and Na+-selective liquid ion-exchanger microelectrodes were used to measure intra-cellular K+activity (ak) and intracellular Na+activity (alj.) of sheep cardiac Purkinje strands in different solutions. In Tyrode's solution with an extracellular K+concentration ([K+]o) of 5.4 mM, ak was between 80 and 140 mM and averaged 109.6 ± 4.0 mM (mean ± SE, 20 strands). The measured ak was closely correlated with the resting membrane potential, so that the K+equilibrium potential was always about 10 mV more negative. When [K+]owas lower than 5.4 mM, ak fell, and when [K+]owas higher than 5.4 mM it increased, ajj. was between 4 and 12 mM, and averaged 6.6 ± 0.6 mM (14 strands). Its variation was also correlated with resting potential. Over a wide range of [K+]oand extracellular Na+concentrations ([Na*],), the af,achanges were such that Na* equilibrium potential remained between +70 and +80 mV. The quiescent membrane behaved as a K+-electrode when fK+lowas higher than 5.4 mM. When [K+]owas low and [Na+]owas zero, then Ca2+and perhaps Cl— contributed to the resting potential.Circ Res 47: 692-700, 1980