SUMMARYCirculating levels of immunoreactive inhibin (ir‐inhibin) and its disappearance after delivery of the placenta were determined in seven pregnant women at term. Serum cestradiol (E2) and progesterone (P4) levels were measured simultaneously and served as comparisons. Fetal contributions of ir‐inhibin were asessed by determining concentrations in the umbilical artery (UA) and vein (UV). Relative changes in circulating levels of ir‐inhibin, E2, and P4 were compared to levels found in nonpregnant women during the early follicular phase (EFP) and mid‐luteal phase (MLP) of the normal menstrual cycle. In pregnant women, ir‐inhibin levels at delivery were 15‐ and 3‐fold higher than EFP and MLP values respectively. The disappearance of all three hormones after removal of the placenta followed a bi‐exponential curve with an initial, rapid component and a second, slower component. There was a highly signifiant positive correlation between the disappearance curves of all three placental hormones (r = 0–97, P>013001). Concentrations of ir‐inhibin in the cord blood were about half that in maternal serum and without significant difference between