Systematic investigations on time, site and mechanism of the periderm development are not yet available, although this superficial layer also appears to be of morphogenetic importance. Therefore, we investigated the surface epithelium (epidermis) of mouse embryos of days 9-12 of gestation using light- and electron-microscopic techniques. On early day 9 it is one-layered. On late day 9 the periderm starts to develop in the region of the upper limb buds, on early day 10 in the region of the lower limb buds, on late day 10 in the back and head regions, and on early day 11 in the ventral region. On day 11 the surface of the limb buds is completely covered by a periderm, in the other regions the periderm development is not yet completed on day 12. The formation of this surface layer starts in many regions and from many cells. The cells form flat, squamous processes that push over neighbouring cells. Subsequently, they detach from the basal lamina, migrate to the surface and adopt an endothelium-like shape. In the contact areas of the periderm cells we find almost exclusively tight junctions. From these findings it can be concluded that the periderm does not have a general protective function against the amniotic fluid, but that it initially protects against regions with a morphogenetically 'active' basal epithelium. A linkage at the basal layer via desmosomes and a well-developed cytoskeleton speak for an additional mechanical function. The occurrence of a protein-synthesizing and secreting apparatus suggests an influence on the composition of the amniotic fluid. A resorbing activity cannot be demonstrated with morphological techniques.