The total protein concentration of fluids in different compartments of the rabbit conceptus from 20 to 28 days of age was measured, and concentrates of these fluids analysed electrophoretically in order to determine their protein composition. For comparison, similar analyses of foetal sera, maternal sera and, where possible, uterine fluid were carried out. The allantoic fluid had a protein concentration less than that of the amniotic fluid, whilst the exocoelomic fluid had a protein concentration greater than that of the amniotic fluid. The total protein concentration of these fluids increased from about 23 days onwards. Compared to the other fluids, protein became much more concentrated in the foetal stomach contents. When analysed by fluid agar electrophoresis, the foetal exocoelomic, amniotic and allantoic fluids and stomach contents contained similar electrophoretic components (albumin, $\alpha $-1-globulin, $\alpha $-2-globulin, $\beta $-globulin and $\gamma $-globulin) and resembled each other in the proportion of the components. These proportions (as best exemplified in the amniotic fluid) changed during the period investigated, and at all times were different to those of the foetal and maternal sera. In certain respects these proportions more closely resembled those of the proteins in the uterine fluid. Analysis of the foetal fluids, both by starch gel electrophoresis and immunoelectrophoresis, revealed a paucity of lipoproteins and of $\alpha $-1- and $\alpha $-2-macroglobulins. Some samples of uterine fluid showed a similar paucity of these components. An $\alpha $-2-globulin component, common to the foetal exocoelomic, amniotic and allantoic fluids, stomach contents and serum, but absent from the maternal serum, was detected (with a rat antiserum to the combined foetal amniotic fluid and stomach contents). Additional components were detected in the foetal stomach contents, both by the rat antiserum and by starch gel electrophoresis. Experiments on the permeability of the membranes in vitro were performed. No obvious increase in permeability of the amnion and yolk sac splanchnopleur with increasing gestational age could be found when maternal serum was dialysed across the isolated foetal membranes. The proportions of the protein components in the dialysates were similar, indicating that no alteration in permeability of the membranes had occurred. The paraplacental chorion, as compared to the yolk sac splanchnopleur, showed a greater permeability to protein in vitro. The role which the uterine fluid and foetal serum might play in governing the protein composition of the foetal fluids is discussed. It is suggested that the paraplacental chorion is the major site for the passage of proteins to the foetal fluids other than the blood.