Upper limb buds of mouse embryos (day 11 + 3 h of development) were cultured for 6 days. During this time the epidermis develops from a two-layered stage, consisting of a basal cell layer and a periderm cell layer, to a multilayered squamous epithelium with a stratum granulosum and a stratum corneum. To investigate the cell kinetics of epidermis and periderm during epidermogenesis the limb buds were labelled with 3H-thymidine at different stages of development. The migration of labelled cells was studied on day 3 in vitro. In the first period of development, before a stratum granulosum has differentiated, each individual cell layer of the epidermis has a cell cycle of its own, i.e. once it has developed each cell layer grows independent of the other. The switching from horizontal to vertical proliferation starts on day 4 of culture with the appearance of the stratum granulosum and is completed on day 5 when a corneal layer begins to develop. With the appearance of the stratum corneum the limb bud shows the typical proliferation of the adult epidermis, which is regenerating only from the basal layer. The labeling behaviour of periderm cells also shows that these cells have a cell cycle of their own and are not formed by cells migrating from the epidermis in an upward direction.