Simultaneous determination of unconjugated 16 alpha-hydroxypregnenolone (16 alphaOH-Preg), 16 alpha-hydroxyprogesterone (16 alphaOH-Prog) and 16 alpha-hydroxydehydroepiandrosterone (16 alphaOH-DHEA) in fetal and neonatal plasma was performed utilizing a newly developed radioimmunoassay. In all neonates, the three 16 alpha-hydroxysteroid levels were consistently higher in umbilical cord plasma than in the maternal peripheral circulation. 16 alpha-OH-Preg in the umbilical arterial plasma increased from 11.2 +/- 3.1 at 24 weeks to 29.7 +/- 12.0 ng/ml at term, 16 alphaOH-Prog from 15.5 +/- 3.2 to 34.3 +/- 11.0 ng/ml and 16 alphaOH-DHEA from 5.1 +/- 1.2 to 5.9 +/- 1.0 ng/ml. In the anencephalic neonates, only 16 alphaOH-Preg showed an increase pattern under ACTH priming. 16 alpha-OH-Preg levels for normal full term neonates remain relatively constant at the first 24 hr and show a slight decrease at 3 days post partum. In small full term neonates, 16 alphaOH-Preg levels in umbilical arterial plasma are considerably higher than in normal neonates and remain at roughly equivalent levels for the first 5 days post partum. 16 alphaOH-Prog and 16 alphaOH-DHEA levels in umbilical arterial plasma in normal and small full term neonates are almost equal and both groups show a rapid decrease during the first 24 hr. Comparison with findings of the three 16 alpha-hydroxysteroids in fetal and neonatal plasma is discussed.