The examinations presented are based on earlier histological, histochemical, ultra-histochemical and biochemical studies about regional differences of placental villi in the center and the periphery of the placentons (materno-fetal circulation-units) of mature human placentas. In this study the question was followed up whether the morphological "immature" imposing villi lying in the centers of the circulation-units are immature like the villi of placentas of earlier stages of pregnancy also referring their proliferation. Autoradiograms were made from specimens of placentas of 38.-41. weak of uncomplicated pregnancies. The degree of nuclear labeling was higher in the villi situated in the center of the placentons. By these findings the centers of the placentons are interpreted to be not areas of lower metabolism - possibly conditioned by maternal blood flow - but regions of growth and regeneration. The results comply well with our up to date conception of formation and growth of the placentons.