Placental steroidogenesis and the interplay between fetus, placenta, and the mother are discussed. The syncytial trophoblastic cytoplasm is the site of elaboration of placental steroids and protein hormones. In addition to the location of the steroid synthesis in the placenta, topics covered include: biogenesis of estrogens, biogenesis of gestagens, reductive metabolism of progesterone, and placental pathology in relation to placental steroidogenesis. Examples of a possible clinical application of the knowledge which has emerged from the results of an experimental analysis of the feto-placento-maternal unit are presented. The helpfulness of estriol determinations in the supervision of endangered pregnancy is well known. The estriol assay gives a fairly reliable parameter for the assessment of fetal well-being or distress and provides valuable prognostic and therapeutic information. It corresponds very well with other clinical parameters and has had a very favorable influence on perinatal statistics. Since the simple static hormone determinations have some disadvantages, a dynamic test of feto-placental function was elaborated by administering steroid hormone precursors to the mother and measuring the additional steroid hormone excretion in comparison with the control values calculated as a percentage of the dose of the precursor administered. Pregnenolone sulphate has been used with success. Injection of the estrogen precursor dehydroepiandrosterone sulphate (DHA-S) was also used as a test of the feto-placental function with encouraging results. Injections of estrogens into the amniotic fluid have been tried as a test for feto-placental function but this approach seems less promising. With a greater knowledge of what ACTH stimulation and corticosteroid inhibition are really doing in the feto-materno-placental system and how the changes have to be interpreted, further refinement of ACTH and corticosteroid test methods may be possible.