Free calcium ion concentration (mmol/l) and pH were determined in whole blood using a semiautomatic electrode system (ICA-1 Radiometer, Copenhagen, Denmark) in 37 normal women, 90 pregnant women (30 from each trimester of gestation), 28 mothers at delivery and their respective newborns. The blood samples from normal controls, pregnant women, umbilical cord and 40-50-hour-old infants were collected anaerobically in vacuum tubes. Duplicate samples drawn from newborns shortly after birth by heel puncture were collected in special heparinized capillary tubes. We observed that Ca2+ concentrations in the second (1.20 +/- 0.04) and third (1.20 +/- 0.05) trimesters of pregnancy, and at delivery (1.18 +/- 0.05), were lower than in the control group (1.23 +/- 0.04). The [Ca2+] in samples from the umbilical vein (1.44 +/- 0.11) and artery (1.45 +/- 0.08) and from newborns 2-5 min after birth (1.34 +/- 0.12) was greater than in control samples. The [Ca2+] in newborns 40-50 hours after birth was lower (1.16 +/- 0.14) than in the control group.