Plasma immunoreactive ACTH (I.R.ACTH), immunoreactive human chorionic somatomammotrophin (I.R.HCS) and 11-OH steroids levels were measured during normal and pathological pregnancies. Plasma I.R.ACTH levels were found to be above normal during pregnancy, with a slight decrease near term. This observation correlates well with the recent description of human chorionic corticotrophin (HCC), i.e. a factor of placental origin possessing a high biological adrenocorticotrophic activity and presenting partial immunological cross-reaction with human ACTH. The 11-OH steroids levels, as measured by a fluorimetric method, increased regularly during pregnancy but, unlike ACTH, reached their highest value near term, I.R.HCS increased progressively until term when a slight decrease was observed. In pathological pregnancies, I.R.ACTH levels behaved like other placental hormones. On the contrary, 11-OH steroids levels remained generally unmodified. Treatment with high doses of prednisolone caused no inhibition of the plasma I.R.ACTH and of plasma 11-OH steroids levels. These data suggest placental autonomy of the secretion of HCC and low adrenal responsiveness to endogenous ACTH variations throughout pregnancy.