(1) Fetal rat skin of 17-day-old embryos was cultured up to 8 days in vitro.(2) Within 3–4 days a complete adult epidermis was formed in vitro. The stages that lead to this formation and the time necessary for it were slightly different in vitro and in vivo. No skin adnexa were formed. (3) Actinomycin D (at 0.02 μg/ml) and puromycin (at 0.1 μg/ml) failed to inhibit the differentiation and the morphological effect of it: i.e. stratum corneum formation. At higher doses a total degeneration of the stratum corneum occurred. (4) It was clear from the incorporation experiments with 3H-thymidine that a labelled component released from DNA breakdown in the stratum granulosum was re-used for DNA synthesis in the nuclei of the basal cell layer.