Continuous cholecalciferol supplementation with up to11,000 IU/d produces a stable serum cholecalciferol concentration in approximately 3 weeks. However, cholecalciferol supplementation of lactating mothers with 6400IU/d produced a continued upward trend in breast milk antirachitic activity at 6 months, but the increase in infant 25(OH)D concentrations did not differ from that of infants supplemented with cholecalciferol at 300IU/d. Further research is needed to determine how frequently intermittent doses should be administered to prevent decreases in breast milk cholecalciferol concentrations below levels necessary to maintain adequate 25(OH)Din infants.