The morphological differentiation of the elements contributing to the chorioallantoic placenta of the rat was studied between days 6 and 16 of gestation, and DNA synthetic activity was investigated in animals killed 1 hour after injection of tritiated thymidine. The chorionic lamina is formed from the epithelial cells lining the ectoplacental cavity, though it is possible that a contribution is made by ectoplacental cone cells at the mesometrial pole of the cavity. From day 8 onwards a high proportion of cells was labelled in both cone and lamina, with the labeliing index reaching a peak of about 80% at 10-11 days. There was no corresponding peak in mitotic activity, which remained at a high level in all regions up to day 12. The amount of thymidine incorporation, as judged by grain count per nucleus, varied. At 9 days a significantly higher grain count was obtained in a central core of cells in the ectoplacental cone, and at day 10 the grain density of labelling in the cone was very much heavier than in the lamina. This difference in labelling intensity was maintained until day 12, and there was no evidence of any influx of heavily labelled cells from the cone into the developing labyrinth.