Corticosteroids (CS) are essential for fetal organ maturation; yet, knowledge of endogeneous CS and precursor levels throughout fetal life is limited. Therefore, unconjugated aldosterone (Aldo), corticosterone (B), 11-deoxycorticosterone (DOC), progesterone (P), 17ahydroxyprogesterone (17-OHP), 11-deoxycortisol (S), cortisol (F), and cortisone (E) were simultaneously determined by RIA after automated Sephadex LH-20 chromatography in 70 control samples of amniotic fluid (AF) obtained at all gestational ages between 14–42 weeks. Levels of the progestins P and 17-OHP slowly increased from means (±SE) of 14.7 ± 2.8 and 1.63 ± 0.21 ng/ml, respectively, in early gestation to maximum levels of 32.4 ± 3.5 and 3.80 ± 0.74 ng/ml at 36–38 weeks (P < 0.005), then dropped significantly (P < 0.01) to 19.2 ± 2.2 and 1.58 ± 0.22 ng/ml at term. All CS levels except E rose very markedly by 3-to 12-fold (P < 0.0001) from the weeks 14–16 (DOC, 0.44 ± 0.08; B, 1.49 ± 0.23; Aldo, 0.043 ± 0.012; S, 0.51 ± 0.10; F, 5.96 ± 0.93 ng/ml) until the 36–38th weeks (DOC, 3.50 ± 0.66; B, 4.60 ± 0.78; Aldo, 0.530 ± 0.109; S, 6.00 ± 0.75; F, 60.8 ± 8.9 ng/ml). Term levels were significantly reduced (P < 0.01) in the less active CS DOC (0.51 ± 0.07 ng/ml),B (2.35 ± 0.35 ng/ml), and S (1.14 ± 0.14 ng/ml), whereas those of thebiologically most potent CS Aldo and F declined less markedly (0.272 ± 0.053 and 23.0 ± 0.75 ng/ml, respectively, at 39–42 weeks). Levels of the inactive glucocorticoid E rose from 8.83 ± 1.08 ng/ml at 14–16 weeks to 16.8 ± 2.6 ng/ml at 31–35 weeks (P < 0.01), then remained rather constant around 11.5 ng/ml until term. It is concluded that afterthe 25th week, largeamounts of biologically active CS areavailable in AF which probably directly inducethe final epithelial maturation of fetal lungs and intestinal tract.