Upon injection of homologous and heterologous serum globulin to the uterine lumen in the rabbit, selective uptake of protein occurs into the foetal circulation, but this accounts for a small fraction only of the material injected. In order to determine whether selection takes place by exclusion of the heterologous molecule at the cell surface or by other means, the present work was undertaken to attempt to measure the loss from the uterus through both maternal and foetal channels. It was found that, whereas the oestrous uterus does not absorb protein from the lumen, the non-gravid uterus in pregnancy does take up a fraction of the dose injected, with appearance of labelled protein in the maternal bloodstream. In the gravid uterus both rabbit and bovine globulins are almost entirely absorbed. It is suggested that these proteins are taken up by the foetal cell equally; a small part, varying in magnitude with the species of origin, is passed on into the foetal circulation as protein and the remainder is broken down.