Blood gas levels, pH, and lactate concentration were studied in fetal heart blood on 70 occasions in 26 fetuses with erythroblastosis. A decreasing hemoglobin concentration correlated to a fall in pH (p = 0.006) and bicarbonate (p = 0.015) and to a rise in base deficit (p = 0.002) and lactate concentration (p = 0.05). Twenty of the fetuses underwent 63 intracardiac transfusions. A multivariance analysis for hemoglobin concentration, percentage of fetal to adult blood, and different acid-base parameters revealed a significant correlation between the presence of adult blood and a rise in PO2 (p = 0.047), but failed to demonstrate any correlation to the other studied acid-base variables. Thus the present study suggests that the fetus can compensate for the physiologic properties of adult hemoglobin that are disadvantageous to itself by increasing PO2 and thereby maintaining oxygen supply. The physiologic background and the clinical implication are discussed.