Plasma clearance, volumes of distribution, and renal and extrarenal clearances of digoxin were calculated from plasma digoxin concentrations and urinary excretion of digoxin after intravenous injection of digoxin in 8 subjects. The investigation was repeated in the same subjects during long-term treatment with spironolactone. Increased plasma concentration of digoxin was detected during spironolactone treatment. Calculated plasma and renal clearances of digoxin and the volumes of distribution decreased statistically significant. Near maximal capacity for the tubular secretion of digoxin was found when normal digoxin dosage was used. It is suggested that unless spironolactone decreases the myocardial sensitivity for digoxin, the loading dose as well as the maintenance dose of digoxin should be reduced during treatment with spironolactone.