Placental growth studies in man and other mammals show that placental mass increases in spite of diminishing rates of cell replication as pregnancy progresses, implying that the weight of the human placenta increases linearly while DNA synthesis is diminishing. The present work utilizes measurement of nuclear DNA synthesis by radioautography. Placental specimens from 44 normal human pregnancies were collected within 1 hour after delivery and autoradiographed by incubation with culture medium and3H thymidine. DNA synthesis was expressed as number of labeled cells/1000 in the mesenchyme and the trophoblast cell populations of the placental villus vs weeks of gestation. The data support the characterization of early placental growth as being hyperplastic, and that of later placental growth as being primarily hypertrophic.