The secretion of human chorionic gonadotropin (hCG) varies quantitatively as well as qualitatively throughout pregnancy. There have been no systematic studies of the placental content of free hCG subunits. Human placentas were obtained from normal term pregnancies or elective abortions and analyzed for altered forms of hCG and its subunits. The tissue was extracted and gel filtered on a calibrated Sephadex G-100 column. Each fraction was radioimmunoassayed for hCG, hCGα and hCGβ in respective homologous radioimmunoassay systems. All placental extracts contained hCG and hCGα; however, the concentrations of those two glycoproteins varied qualitatively and quantitatively with progression of pregnancy. The absolute amount of hCG and hCGα decreased after the first trimester but the relative quantity of hCGα exceeded that for immunoreactive hCG by more than 10-fold in the last two trimesters. In addition to native hCGa, another molecular species of hCGα became apparent in the second trimester. The significance of that form is yet to be ascertained.