Fetal blood samples were obtained fetoscopically from 32 Rh isoimmunized pregnancies at 18-32 weeks' gestation, and the hemoglobin concentration, plasma lactate concentration, and oxygen content were measured. When the hemoglobin concentration was more than 8 g/dL, the umbilical arterial and venous lactate concentrations were equal. Abnormal elevations of lactate were found in the umbilical artery at hemoglobin concentrations below 8 g/dL (oxygen content 2 mmol/L) and in the umbilical vein at hemoglobin concentrations below 4 g/dL (oxygen content 2 mmol/L); the arterial lactate values were higher than the venous. These results show that lactate is produced by the human fetus stressed by anemic hypoxia and suggest that compensatory cardiovascular mechanisms are unable to maintain adequate oxygenation to all tissues when the umbilical venous oxygen content falls below 2 mmol/L.