“Free” (solvent-extractable) and “conjugated” (acid-hydrolyzable) estriol levels and their sum as total estriol were determined in paired samples of intervillous space and maternal arm vein plasma and in paired samples of umbilical artery and vein plasma. Estriol levels were determined by the method of Roy and Brown. Free and total estriol levels in the intervillous space plasma were significantly higher than those found in the maternal arm vein plasma. This suggests the transfer of estriol from the placenta to the mother and might alone account for the high urinary estriol levels found in pregnancy. No differences between free, conjugated or total estriol values were found in paired umbilical artery and vein plasma samples.