The distribution of digoxin in the myocardium, skeletal muscle, erythrocytes, and plasma (or serum) was studied in 19 infants. There was a linear relationship between myocardium and serum concentrations and no saturation was observed over the serum concentration range of 0.5-8.6 ng/ml. Myocardium uptake of digoxin was nearly twice as great in infants as in adults at any given serum concentration. Erythrocyte: plasma concentration ratios of digoxin were one-third smaller during digitalization than during maintenance digoxin therapy. The latter ratios were also three times greater in infants than found previously in adults. Their findings are consistent with a greater apparent volume of distribution of digoxin in infants and may partly explain the unusually large therapeutic doses needed in infants.