1. Maternal blood was made transiently hypertonic by rapid I.V. infusion of a concentrated mannitol solution into pregnant ewes bearing lambs with an indwelling flow sensor and vascular catheters. 2. The transplacental flows of water and of Na+ and Cl‐ were calculated from the umbilical arteriovenous differences in the concentrations of 125I‐labelled albumin and electrolytes, and the fetal placental blood flow. 3. The reflection coefficients of Na+ and Cl‐ were calculated by means of the Patlak equation and found to be 0.85 +/‐ 0.04 and 0.68 +/‐ 0.04 (means +/‐ S.E.M.). The filtration coefficient was 1.02 x 10(‐7) +/‐ 0.12 x 10(‐7) cm5 dyne‐1 s‐1. 4. The results fitted best to an equivalent pore radius in the placental barrier smaller than the currently accepted 0.44 nm but not less than 0.35 nm.
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