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1. The rate of appearance in uterine venous blood of radioactively labelled, polar, non-electrolytes has been measured following their injection into the foetal circulation in the chronically catheterized sheep near term. Uterine blood flow was measured by an antipyrine technique. 2. Estimates of a placental permeability constant not corrected for area were 61-2 +/- 5-5 ml. min-1 (mean +/- S.E. of mean) for [14C] urea, 1-85 +/- 0-16 for [14C]erythritol and 0-21 +/- 0-03 for [3H]mannitol. Results are also presented for [14C]ethylene glycol, [14C]L-glucose, [14C]mannitol, [51Cr] EDTA and [3H] and [14C]sucrose. 3. In four sheep, permeability measurements for several solutes were made and the results were analysed in terms of restricted diffusion via cylindrical, water-filled pores. Calculation of pore radius was made by a minimum variance method and values ranging from 0-43 to 0-45 nm were obtained. 4. Stability and absence of protein binding of probe molecules was investigated by gel permeation on Sephadex columns.