Activation of the Na+-activated K+ channels (KNa channels) has been suggested to contribute to the ischaemia-induced accumulation of extracellular K+ (K+e) in the mammalian myocardium. Recent evidence shows that these channels are not present in rat ventricular myocytes [9]. We have therefore investigated the effect of raised intracellular Na+ activity (aiNa) on intracellular K+ activity (aiK) in guinea-pig myocytes, which possess the channels, and on rat ventricular myocytes which do not. The Na+-activated K+ current was activated by an increase in aiNa induced by removing extracellular Ca2+ and Mg2+ and inhibiting the Na-pump. The aiNa increased and the aiK decreased in both guinea-pig and rat myocytes superfused with Ca2+- and Mg2+-free Tyrode. The new steady-state increase in aiNa and decline in aiK were similar in both species. Inhibition of the Na-pump resulted in an additional increase in aiNa and decrease in aiK in both species. However, both the increase in aiNa and decrease in aiK were greater in guinea-pig myocytes and the decline in aiK in guinea-pig myocytes followed the development of a large Na+-activated K+ current. When Li+ replaced Na+ in the superfusate the Na+-activated K+ current did not develop and the fall in aiK was reduced. In Na+-loaded rat myocytes, which do not have a Na+-activated K+ current, the decline in aiK was reduced and blocked by 2 mM Mg2+ suggesting that a Mg2+-sensitive non-specific cation channel may be involved in the K+ efflux from rat myocytes [12]. These data suggest that KNa channels are a major route for K+ efflux from Na+-loaded guinea-pig myocytes.