In 36 newborn infants admitted to the Children's Hospital of Oklahoma, we studied the hypotheses that intravenous bolus infusions of Ca as calcium gluconate over 10 min may (a) result in acute metabolic acidosis, (b) increase serum osmolality, (c) increase serum free bilirubin, and (d) decrease serum phosphorus concentrations. All infants received 18 mg/kg of elemental calcium, as either a 5 or 10% solution of calcium gluconate. Blood ionized calcium (iCa) rose significantly with i.v. bolus Ca infusion (p < 0.005) and blood pH and serum P declined significantly (p < 0.05) with i.v. bolus Ca infusion in infants. Serum free bilirubin was not significantly altered. Serum osmolality rose significantly from baseline with bolus infusion of Ca as a 10% calcium gluconatc solution and did not change significantly with bolus infusion of a diluted 5% solution. In neonates, intravenous bolus calcium infusion (a) decreases blood pH, (b) infusion of 10% but not 5% calcium gluconate increases serum osmolality, (c) serum free bilirubin concentrations were not altered, and (d) serum phosphorus concentrations were decreased.