An investigation into the relations between the daily dose of digoxin, drug regimen, serum digoxin concentration, and creatinine and digoxin clearance was carried out in a group of elderly ambulant patients in domiciliary practice. Moderate to severe impairment of renal function was found both in patients taking digoxin and in elderly control subjects. Plasma digoxin levels were not related to blood urea concentrations or creatinine clearance. Digoxin clearance was less than creatinine clearance. Now that plasma digoxin levels can be measured relatively easily their estimation should become part of clinical practice.