CONCENTRATIONS OF NA- IN TRANSPLACENTAL ULTRAFILTRATE IN SHEEP( AND CL)

1. Maternal blood was made transiently hypertonic by rapid I.V. infusi on of a concentrated mannitol solution into pregnant ewes bearing lamb s with an indwelling flow sensor and vascular catheters. 2. The transp lacental flows of water and of Na+ and Cl- were calculated from the um bilical arteriovenous differences in the concentrations of I-125-label led albumin and electrolytes, and the fetal placental blood flow. 3. T he 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 (m eans +/- S.E.M.). The filtration coefficient was 1.02 x 10(-7) +/- 0.1 2 x 10(-7) cm(5) dyne(-1) s(-1). 4. The results fitted best to an equi valent pore radius in the placental barrier smaller than the currently accepted 0.44 nm but not less than 0.35 nm.