Thin kidney cortex explants from newborn rabbits, consisting of the capsula fibrosa with an adherent layer of nephrogenic material (s-shaped bodies and collecting duct anlagen) were isolated and cultured for various periods (up to 15 days) by conventional procedures. Within the first 24 h of culturing, it was observed that the explants form globular bodies covered by a monolayer epithelium. The cells are cuboidal or cylindrical, and may, in later stages, consist of two or even more cell layers. The epithelium shows a polar differentiation, with deep tight junctions at the apical poles and lateral intercellular spaces opening to the basal surface. A basal lamina is not consistently developed. The support of the epithelium consists of fibroblasts and degenerating tissue elements of the explants. It can be shown that the epithelium is derived from the anlagen of the collecting ducts. Structurally, the epithelium resembles a collecting duct epithelium. The experimental system is considered as a useful model to study metabolism and function of the renal collecting duct epithelium, as well as developmental problems, such as cell migration, outgrowth, and differentiation.