Objectives. The integrity of the parathyroid axis was tested in 18 infants and young children undergoing repair of congenital heart disease with cardiopulmonary bypass. Background. Infants are believed to have an immature parathyroid hormone response to hypocalcemia. Whereas adults are known to respond appropriately to hypocalcemia during cardiopulmonary bypass, children have not been studied carefully. Methods. Calcium, magnesium, parathyroid hormone, phosphate and total protein were measured in blood samples withdrawn at defined times before, during and after cardiopulmonary bypass. Results. At the initiation of cardiopulmonary bypass, ionized calcium decreased markedly in 12 infants ≤ 24 months old (mean ± SEM 1.11 ± 0.04 to 0.29 ± 0.05 mM) and decreased significantly in 6 young children > 24 months old (1.19 ± 0.02 to 0.42 ± 0.12 mM). In response to hypocalcemia, parathyroid hormone concentration increased significantly in both the infants (from 42 ± 8 to 103 ± 29 and 85 ± 22 pg/ml) and the young children (from 39 ± 8 to 44 ± 20 and 92 ± 30 pg/ml). Before separation from cardiopulmonary bypass, increased parathyroid hormone concentration restored ionized calcium concentration to 0.75 ± 0.03 mM in the infants and to 0.92 ± 0.07 mM in the young children. There was no significant influence of either age or the use of deep hypothermia and circulatory arrest on either calcium or parathyroid hormone responses. Total magnesium and total protein concentrations decreased on initiation of cardiopulmonary bypass and thereafter remained stable. Phosphate concentrations were unchanged during the study. Conclusions. In infants and young children undergoing cardiac surgery, the parathyroid hormone response to both hypecalcemia and to rising ionized calcium concentrations was at least as great as that of adults. Thus, the calcium-parathyroid-vitamin D axis functions in infants and young children as it does in adults.