The oestrogen content of two 24 h pools of pregnancy faeces, obtained from 2 normal women in the 33rd-37th week og gestation, was studied. The qualitative analyses were made by gas chromatography - mass spectrometry and the quantitative analyses by mass fragmentography. The presence of the following oestrogens in pregnancy faeces was established: Oestriol, oestrone, oestradiol-17 beta, 16-epioestriol, 17-epioestriol, 16 alpha-hydroxyoestrone, 16-oxo-oestradiol-17 beta, 15 alpha-hydroxyoestrone and 15 alpha-hydroxyoestradiol-17 beta. In addition, mass fragmentographic evidence was obtained for the presence of 16 beta-hydroxyoestrone, 2-methoxyoestrone and oestradiol-17 alpha. The total oestrogen excretion determined in the two pools was 786 and 1300 mug per 24 h. Unconjugated oestrogens accounted for 97.8 and 98.6% of these amounts, respectively. Oestriol, oestradiol-17 beta, 15 alpha-hydroxyoestradiol-17 beta, 16-epioestriol and oestrone, in that order, were quantitatively the most significant of the oestrogens determined. The remarkably high levels of oestradiol-17 beta fround in faeces show, that in pregnancy, this mode of excretion is as important as urine for the elimination of this biologically active steroid. It is suggested that some of the oestradiol may have b-en formed through bacterial enzyme action from other oestrogens or neutral steroids. Only trace amounts of ring D alpha-ketolic oestrogens were found in faeces. This is in marked contrast to the considerable amounts of these steroids found in pregnancy bile and urine.