Serum digoxin concentrations were measured by radio-immunoassay in seven elderly patients during digitalisation with digoxin 0.25 mg daily, without a loading dose. Serum concentrations in the therapeutic range (1–2 ng/ml) were reached in four days in five patients with normal renal function, while ‘toxic’ levels of 3 ng/ml were reached in the same time in two patients with renal impairment. Digoxin levels were measured after a single dose of 0.0625–0.25 mg in 16 digitalised patients, and the frequency of ventricular ectopic beats determined from continuous electrocardiographic records. Digoxin absorption was normal, peak serum levels, often over 3 ng/ml, being reached after 90 minutes. An increased frequency of ventricular ectopic beats was observed in three of four patients given digoxin 0.25 mg, but only three to four hours after peak serum levels.