Experiments were performed on unanaesthetized ewes in the last third of pregnancy. Fetuses and ewes had indwelling vascular catheters. In some of the experiments fetal urine was drained to the exterior by means of an indwelling vesicular catheter. Placental diffusion permeabilities were measured with 22Na+ and 36Cl- injected into eight fetuses. Volumes of distribution of Na+ and Cl- in the conceptus were Na+: 548 +/- 24, Cl-: 760 +/- 51 (ml/kg fetal wt +/- SEM). Diffusion permeabilities were Na+: 5.2 X 10(-3) +/- 0.3 X 10(-3), Cl-: 9.8 X 10(-3) +/- 0.9 X 10(-3) (ml.s-1.kg-1 +/- SEM). Ultrafiltration-reflection-coefficients of Na+ and Cl- in the placental exchange barrier were measured 17 times in seven fetuses with vesicular catheters. The transplacental e.m.f. was estimated from these results, on the assumption that the individual reflection-coefficients of Na+ should correlate with those of Cl-. The best estimate was -1.0 mV (fetus negative), and the best estimates of the placental reflection-coefficients were Na+: 0.83 and Cl-: 0.79. There was a reliable negative correlation (P < 0.01) between the calculated reflection-coefficients and the osmolality of the urine secreted by the fetus. This suggested that the concentration of vasopressin in fetal plasma affects the membrane characteristics of the placenta. The mean total osmotic force across the placental barrier of the sheep in these experiments was calculated to be 260 Pa (2 mmHg).