The Bohr Principle via continuous indirect calorimetry was used to estimate human fetal-placental unit metabolic rate in 12 normal women undergoing elective caesarean section under continuous lumbar epidural anaesthesia. Maternal oxygen consumption decreased after umbilical cord clamping and after placental removal. Fetal-placental unit oxygen consumption was 10.7+/-1.3 ml/min per kg (mean+/-SEM). Fetal oxygen consumption was 6.8+/-1.4 ml/min per kg. Placental oxygen consumption was 37+/-12 ml/min per kg. Fetal-placental unit carbon dioxide production was 9.2+/-1.2 ml/min per kg. These mean values agree favourably with measurements of uterine and fetal metabolism from other mammalian species. Maternal minute ventilation decreased with removal of the fetal-placental unit, and this decrease was found to be linearly related to the fetal-placental unit carbon dioxide production.