Plasma levels of unconjugated oestrone,' oestradiol-17p and oestriol from the 22nd week of uncomplicated human pregnancy until delivery have been estimated by means of radioimmunoassays. In some cases, the oestrogens were separated by Sephadex LH-20 chromatography before the radioimmunoassays. Plasma levels of unconjugated oestrone increased from a mean of 3 ng/ml in the 22nd week to 9.9 ng/ml in week 41. Oestradiol-17p concentrations in plasma were estimated by two different radioimmunoassays. With the most specific method, plasma mean concentrations were found to rise from 7.5 ng/ml in the 22nd week to 23.6 ng/ml in the 38th week. A somewhat less specific assay with a crossreaction of 50 per cent for oestrone gave a mean level of 8.3 ng/ml in week 22 increasing to 26.2 ng/ml in week 41. When unconjugated oestradiol-17p in plasma was estimated after Sephadex LH-20 chromatography somewhat lower concentrations were found. ' The following abbreviations and trivial names are used: oestrone = 3-hydroxy-1,3,5( 10)-oestratrien-1 7-one; oestradiol-17P = 1,3,5( IO)-oestratriene-3,17P-diol; oestriol = 1,3,5( lO)-oestratriene-3,16a, 17P-triol; dehydroepiandrosterone = DHA = 3P-hydroxy-5-androsten-17-one; dehydroepiandrosterone sulphate = DHAS = 3P-sulphoxy-Sandrosten-1 7-one; 16a-hydroxydehydroepiandrosterone = 16a-OH-DHS = 30, 16a-dihydroxy-5-androsten-17-one.The results for the estimation of unconjugated oestriol in plasma were similar whether or not the sample was separated by Sephadex LH-20 chromatography. The mean level of 2.9 ng/ml in week 22 increased to 14.3 ng/ml in week 41. The range of all the studied oestrogens increased towards term. The quotients (oestrone + oestradiol-17p)/oestriol, and oestradiol-17p/oestriol decreased towards the end of pregnancy.Individual patients tested regularly during the latter part of pregnancy showed a characteristic pattern for all three oestrogens with rather small deviations from the original relative level within the normal range, but with large fluctuations towards the end of pregnancy.Significant variations in the plasma levels of unconjugated oestradiol-17p and oestriol were noted during a 24-hour period and during shorter periods. However, no systematic circadian rhythm was found.