Abstract
1. The transfer of iron from maternal plasma to the foetus was studied in the cat using cat transferrin labelled with (59)Fe and radioiodine.2. Pregnancy was accompanied by a fall in maternal haematocrit, a rise in serum total iron-binding capacity and iron stores, but no change in serum iron concentration or plasma iron turnover.3. Near-term foetal haematocrit and serum iron concentration were higher and iron binding capacity lower than in the pregnant animals.4. The uptake of (59)Fe from maternal plasma by the placenta and transfer to the foetuses were slow and small in amount. The daily transfer of iron to the foetuses calculated from these data was only 5 mug.5. There was a small, transient uptake of (59)Fe from the plasma by the foetal membranes.6. Small amounts of plasma transferrin and albumin were slowly taken up by the placenta.7. It was concluded that the maternal endothelium of the placenta of the cat acts as a barrier for the uptake of plasma transferrin-bound iron. As a result the rate of iron transfer to the foetuses from maternal plasma is inadequate to account for their rate of iron accumulation, and another source, such as maternal erythrocytes, must supply most of the iron to the foetuses.