The permeability of the rat placenta at 21 days' gestation (term is 23 days) has been measured with fetuses intact in situ and by perfusion of the fetal circulation. Intact measurements were made by removal of fetuses at known time intervals after injection of radioisotopes to the anaesthetized mother and measurement of their radioactivity content. Unidirectional clearance (Kmf) was calculated and found to be in proportion to the diffusion coefficient in water (Dw) for [14C]mannitol, [14C]sucrose, [51Cr]EDTA and [3H]inulin but was relatively high for 22Na and low for [125I]albumin. The high value for 22Na is partially explained by transfer across the yolk sac placenta; cautery of the vitelline vessels significantly reduced the Kmf for 22Na but not that for [51Cr]EDTA or [125I]albumin. The low [125I]albumin value is explained on the basis of restricted diffusion of this tracer. Perfused permeability measurements were made by injection of radioisotope into the anaesthetized mother and measurement of the radioactive content of fetal perfusate samples. Although the Kmf for Na+ was significantly (P less than 0.01) reduced by 25 per cent, there was no other significant difference between results with the perfused placentae compared with those from the intact placentae. Comparison of the intact data with similar measurements in guinea pig and human confirms the similar permeability properties of the haemochorial placentae as a whole; the extra trophoblast layers in the rat make no discernible difference.