Ten healthy male volunteers each received 0.5 mg digoxin orally and i.v. in a randomised, cross-over sequence with at least two weeks between doses. Plasma concentration and cumulative urinary excretion of digoxin were measured up to 6 and 144 h, respectively, after administration using a radioimmunoassay method. Absolute bioavailability (i.e. the percentage absorption from tablets compared to i.v. injection) was calculated by four methods: by comparing areas under plasma concentration/time curves (AUC) up to 6 h and to infinity, also by comparing cumulative urinary excretion up to 144 h (t max.) and to infinity. The mean of the two extrapolated values for the absolute bioavailability of digoxin (Sandoz) tablets is 78%.