Aldosterone upregulates the Na+-K+ pump in kidney and colon, classical target organs for the hormone. An effect on pump function in the heart is not firmly established. Because the myocardium contains mineralocorticoid receptors, we examined whether aldosterone has an effect on Na+-K+ pump function in cardiac myocytes. Myocytes were isolated from rabbits given aldosterone via osmotic minipumps and from controls. Electrogenic Na+-K+ pump current, arising from the 3:2 Na+:K+ exchange ratio, was measured in single myocytes using the whole-cell patch clamp technique. Treatment with aldosterone induced a decrease in pump current measured when myocytes were dialyzed with patch pipette solution containing Na+ in a concentration of 10 mmol/L, whereas there was no effect measured when the solution contained 80 mmol/L Na+. Aldosterone had no effect on myocardial Na+-K+ pump concentration evaluated by vanadate-facilitated [3H]ouabain binding or by K+-dependent paranitrophenylphosphatase activity in crude homogenates. Aldosterone induced an increase in intracellular Na+ activity. The aldosterone-induced decrease in pump current and increased intracellular Na+ were prevented by cotreatment with the mineralocorticoid receptor antagonist spironolactone. Our results indicate that hyperaldosteronemia decreases the apparent Na+ affinity of the Na+-K+ pump, whereas it has no effect on maximal pump capacity.