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1. In anaesthetized sheep near term the O₂ uptake of the foetus from the placenta was reduced to zero either by replacing the foetus with a mechanical pump, or by ventilation of the foetus after delivery or in utero. When umbilical arterial and venous P_O2 were made equal, they were much less than maternal arterial P_O2.

2. This difference in P_O2 was attributed to O₂ consumption both within the placental cotyledons and the extra-cotyledonary foetal membranes.

3. The foetal membranes and cord in the sheep are supplied by branches of umbilical blood vessels and constitute an oxygen-consuming by-pass on the foetal side of the placenta.

4. The O₂ consumption of individual placental cotyledons was measured in vivo.

5. The combined O₂ consumption of the whole placenta and foetal membranes in vivo was a considerable fraction of foetal O₂ consumption.

6. The consequences of these observations are discussed in relation to O₂ transfer across the placenta.