Eighty-four fetal membranes--amnion, chorion and adhaerent decidua--were investigated with the in vitro uptake of tritiated thymidine in the last trimester of pregnancy. Labelling patterns were observed in each component of fetal membranes and decidua. The proliferation activity of the cells was determined by the 3H-thymidine labelling index. Significant correlation was found between labelling activity of chorion and decidua. The cells of the chorion layer and decidua showed linear decreasing uptake of tritiated thymidine from 200th to 300th day of pregnancy. The cellular growth of the chorion epithelium seems to be directed from the pseudobasement membrane to the decidua layer. There was also a significantly increased DNA-synthesis in decidua cells when labelled endothelium-cells of decidua vessels were observed. In the same cases we found significant correlation to mean placental weight. When fibrinoid deposits between the chorion and decidua layer were found the mean tritium index of the chorion layer was significantly lower than when fibrinoid material was missing.