K+- and Na+-selective liquid ion-exchanger microelectrodes were used to measure intracellular K+ activity (aK i) and intracellular Na+ activity (aNa i) of sheep cardiac Purkinje strands in different solutions. In Tyrode's solution with an extracellular K+ concentration ([K+]o) of 5.4 mM, aK i was between 80 and 140 mM and averaged 109.6 +/- 4.0 mM (mean +/- SE, 20 strands). The measured aK i was closely correlated with the resting membrane potential, so that the K+ equilibrium potential was always about 10 mV more negative. When [K+]o was lower than 5.4 mM, aK i fell, and when [K+]o was higher than 5.4 mM it increased, aNa i 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+]o and extracellular Na+ concentrations ([Na+]o), the aiNa changes were such that Na+ equilibrium potential remained between +70 and +80 mV. The quiescent membrane behaved as a K+-electrode when [K+]o was higher than 5.4 mM. When [K+]o was low and [Na+]o was zero, then Ca2+ and perhaps Cl- contributed to the resting potential.