A total of 1109 determinations of digoxin concentration in serum were performed in 317 patients with cardiac failure during oral maintenance therapy with digoxin, beta-acetyldigoxin and beta-methyldigoxin. It was shown that the optimal therapeutic serum concentration (1.21 to 1.70 ng/ml) can be obtained reliably if the dosage of digoxin and its derivatives is based on the body weight. The daily doses recommended for oral maintenance therapy are 4mug/kg for beta-methyldigoxin, 5 mug/kg for beta-acetyldigoxin, and 8 mug/kg for digoxin. For initiating cardiac therapy the double maintenance dose can be prescribed once. Digoxin derivatives should be preferred to digoxin when choosing the drug.