Renal clearance of digoxin and creatinine was determined simultaneously on 30 occasions in 28 elderly inpatients on maintenance digoxin therapy. The mean ratio of digoxin to creatinine clearance was 1.04 (± S.E.M. 0.08) Multiple 24-hour urinary excretions of digoxin were measured in 10 patients on digoxin on a dose of 0.25 mg/day, four on 0.125 mg/day, and six on 0.0625 mg/day. Mean urinary excretion was 42% of dose per day. Calculated ‘net biliary clearance’ averaged 18 ml/min. Knowledge of the magnitude of renal and net biliary clearance, together with assumed values for fractional absorption allows a relationship between serum digoxin levels and creatinine clearance to be calculated for each dose level. There was reasonable agreement between observed and expected serum digoxin levels in 41 elderly patients in whom creatinine clearance was measured, and in 106 in whom glomerular filtration rate was estimated from serum urea and creatinine concentrations. Quantitation of the kinetics of digoxin in the elderly can make maintenance therapy more rational and therefore safer and more effective.