SUMMARY1. The permeability of the endothelial cell layer of the foetal capillaries of the rabbit placenta was measured by the tracer-dilution method. Isotopically labelled water, urea, chloride and sodium ions, and inulin were used. Radio-iodinated human serum albumin served as reference tracer. The measurements were made in in situ placentas of 27-29 days gestation of which the foetal circulation was perfused with homologous blood.2. The permeability of water was found to be 1-7 ml. min-. g-l, the permeabilities of urea, chloride ions and sodium ions were in the range of 0-28-0-42 ml. min-'. g-and the permeability of inulin was 0-053 ml. min-' . g-. These values may be underestimates, particularly the value of water.3. The approximate maternal blood space, foetal blood space, and extravascular sodium space were determined in rabbit placentas of 27-29 days gestation and found to be 0-16, 0 07, and 0-48 ml./g respectively.4. The permeabilities of the endothelial layer were compared to those published for the entire placenta and found to be far greater. The resistance to diffusion of the endothelial layer is only a small part of the total resistance to diffusion. Most of the resistance is localized in the syncytioand/or cellular trophoblasts.5. It was concluded that lipid insoluble molecules diffuse through the interstitial water spaces of the trophoblast in few, relatively wide channels but that diffusing water can make use of intracellular as well as extracellular paths.