The vaginal complex of marsupials differs from that of eutherians. Cervices open separately in a sinus vaginalis or cul-de-sac. Two lateral vaginae adjoin the sinus vaginalis and fuse at the level of the urethra opening and form the sinus urogenitalis. During the estrous cycle the vaginal epithelium undergoes a number of specified morphological changes. This paper is the first to describe these changes on an ultrastructural level in a marsupial. Investigations in Monodelphis vagina reveal that a cyclic switch exists between a keratinized and a stratified nonkeratinized epithelium. Keratinization starts during proestrus and reaches its maximum during estrus. In the postestrus, desquamation of the stratum corneum takes place, mostly in two steps. In metestrus one to two additional layers of the now nonkeratinized surface cells are shed into the vaginal lumen. Typical cell structures, such as keratin filaments, keratohyalin and membrane-coating granules, are involved in the keratinization process. Keratohyalin is found in the cytoplasm as well as in the nucleus of stratum granulosum cells, a phenomenon which is known from other parakeratinized epithelia of rapid turnover. Membrane-coating granules, responsible for the permeability barrier between the epithelial cells, are of the nonlamellated type in the nonkeratinized epithelium and produce an amorphous material in the intercellular spaces after extrusion. At periods, however, when the epithelium is keratinized, membrane-coating granules are of the lamellated type and form a lamellated barrier structure after extrusion in the intercellular space. The loss of the protective keratinized layers asks for an additional defense mechanism for the epithelium. The migration of leukocytes through the epithelium predominantly during post- and metestrus and their presence in the vaginal lumen may play a protective role together with the bacterial content.