In normal subjects a relation was found between the log plasma aldosterone concentration (PAC) and the ratio of renal potassium excretion (U<sub>ĸ</sub>) and sodium delivery to the sodium-potassium exchange site (U<sub>Na+ĸ</sub>). This relationship was independent of plasma renin activity (PRA). On these grounds, the U<sub>ĸ</sub>/U<sub>Na+ĸ</sub> ratio disturbance was considered to be a function of the PAC, and this relation was considered to reflect an altered sensitivity of the distal tubule to aldosterone. In several pathological conditions involving the kidney, the relation between the PAC and U<sub>ĸ</sub>/U<sub>Na+ĸ</sub> remained normal while the glomerular filtration rate was within normal limits. Under these conditions, however, the serum potassium concentration had some influence on this relation, in that a low potassium concentration was accompanied by an elevated (though still normal) PAC relative to the U<sub>ĸ</sub>/U<sub>Na+ĸ</sub> ratio. The relation was completely abolished when the effect of endogenous aldosterone was impaired by chronic spironolactone administration in patients with essential hypertension. Analysis to these relations in chronic renal disease made it possible to classify them according to the pathophysiological disturbance in question. We conclude that determination of the U<sub>ĸ</sub>/U<sub>Na+ĸ</sub> ratio and its relation to the PAC, PRA, and serum potassium level is very useful for the analysis of disorders of potassium metabolism.