DNA and protein synthesis have been studied in placental villi from normal and pathological cases by in vitro incorporation of tritiated thymidine or tritiated proline and subsequent counting or autoradiography. It appeared that cytotrophoblastic DNA synthesis continued until term and that it was particularly important in preeclampsia cases and in cases of villous immaturity (rhesus sensitization). Protein synthesis was also increased in preeclampsia and seemed to be due to a very active cytotrophoblastic metabolism. The most interesting finding was that in preeclampsia cases, especially when intrauterine growth retardation was superimposed, villous capillary endothelial cell proliferation was as prominent as in cases where villous maturation was not achieved. Such results are highly suggestive of an important compensatory proliferative mechanism in the placentae of preeclamptic women.