The excretion of potassium and the reabsorption of sodium were studied in isolated perfused kidneys of adrenalectomized rats following the administration of aldosterone. Aldosterone (10-7M) produced a rapid increase in fractional excretion of potassium, without affecting glomerular filtration rate, urine flow rate or sodium reabsorption. Spironolactone (10-4M) completely abolished the kaliuretic effect of aldosterone, while actinomycin D had no inhibitory effect. In the absence of exogenous metabolic substrate, enhancement of potassium excretion was not seen following aldosterone. With either glucose, pyruvate, lactate or α-ketoglutarate in the perfusion medium, the kaliuretic effect of aldosterone was restored. The results confirm the direct action of aldosterone on renal potassium excretion and its dependency upon the availability of specific metabolic substrates.