• Adult serum ionized calcium (iCa) concentrations are higher when using the newer, highly sensitive, ion-selective electrodes compared with older electrodes. Currently used neonatal normative ranges were established using older electrodes and not under standardized conditions or age. Thirty term infants, carefully screened to exclude confounding factors that could affect serum iCa concentration, were studied at birth and 2 and 24 hours of age for serum iCa concentrations. Mean concentrations declined from 1.45 mmol/L (5.82 mg/dL) at birth to 1.33 mmol/L (5.34 mg/dL) at 2 hours to 1.23 mmol/L (4.92 mg/dL) at 24 hours. The 95% confidence limits at 24 hours ranged from 1.10 to 1.36 mmol/L (4.40 to 5.44 mg/dL). Using newer ion-selective electrodes, normal neonatal ranges for iCa concentrations during the first 24 hours of age are higher than published references.