Serum concentrations of total and ionic calcium, magnesium, phosphorus, and albumin were measureed in maternal and cord blood of 115 near-term deliveries. The same measurements (except for ionic calcium) were made in blood obtained from corresponding newborns at 24 hours of age. Cord levels of all components exceeded maternal values, and maternal and cord levels correlated significantly with each other. In the case of calcium, the cord-maternal difference involved both ionic and protein-bound forms. Significant umbilical arterio-venous differences were found only in the case of total calcium, and this difference reflected variation in the protein-bound form only. During the first 24 hours postpartum, total calcium concentration fell (by an average of 0.75 mEq/liter), phosphorus levels rose (by an average of 0.63 mg/dl), and magnesium and albumin did not change significantly. Cord levels of all agents correlated significantly with corresponding neonatal values. In view of the significant positive relationships demonstrated between maternal and cord levels and between cord and neonatal levels, these results substantiate the importance of the maternal serum ionic calcium concentration in normal perinatal calcium homeostasis.