Inactivation of digoxin by reduction of the lactone ring has recently been shown to occur in one third of adults and to be mediated by anaerobic intestinal bacteria. Children from birth through adolescence were studied to determine the pattern of development of this gut flora-mediated process. None of 36 digitalized infants 8 months of age or less excreted reduced digoxin metabolites in the urine. The adult pattern of digoxin reduction product excretion by one third of patients was observed after 16 months of age; however, high levels of digoxin reduction products such as are found in 10% of adults were not encountered in children less than 9 years of age. Even though reduced metabolites were not formed in vivo early in life, stool cultures of 20 of 73 infants younger than 8 months of age contained digoxin reduction product-forming bacteria at high concentrations, in some instances as early as the second week of life. Maturation of the gut flora with respect to digoxin metabolism appears to be a protracted process. The relative digoxin resistance of infants and children is not due to bacterial inactivation.