Eleven primiparae (aged from 20 to 31 years) have been sampled at the time of partum and throughout the puerperium for simultaneous determinations of plasma levels of inactive (prorenin) and active (PRA) renins, aldosterone (PA) and cortisol (PC). Biodata have been compared to time-qualified values obtained in non-pregnant adult women (aged from 17 to 37 years). Pregnant women show peripartum levels of prorenin, PRA, PA and PC significantly higher than the time-qualified reference intervals. Gestational increment is meanly of 3-4 times for prorenin, of 5-8 times for PRA, of 17-19 times for PA, of 4-5 times for PC. Puerperal levels of PRA, PA and PC show a progressive decline while those of prorenin suddenly fall and then remain quite stable. Neonatal levels of prorenin, PRA, PA and PC are higher than normal. Neonates show levels of prorenin, PRA, PA and PC, respectively equal, increased equal and decreased when compared to mothers. The different equipment of hormonal levels in mother and newborn leads to suggest a selective role of the placenta as a filter.