The bioavailability of digoxin (lanoxin) tablets, oral aqueous solution of digoxin, and capsules containing a solution of digoxin was compared with digoxin given intravenously over 1 and 3 hr. The mean peak serum concentration of digoxin after the 1-hr intravenous infusion was 5 ng/ml, after the 3-hr infusion, 3.5 ng/ml, and after the oral solution, 2.0 ng/ml. There was an equivalent bioavailability of the oral solution and reference tablets of digoxin. The digoxin in capsules tended to be better absorbed than the reference tablets. There was 21% more digoxin excreted over 6 days after the 3 hr iv infusion than after the 1 hr iv infusion. This indicates that the calculated bioavailability of an orally administered dose of digoxin may vary with the rapidity of injection of the intravenous standard. It is estimated that an oral tablet of digoxin of 0.5 mg has about the same bioavailability as 0.35 of digoxin given by slow intravenous infusion (or 0.4 mg if calculated against a rapid intravenous injection).