In the fetal guinea pig, transitional epithelium develops at a little beyond mid-term of gestation from a single layer of columnar cells. Some of the cells elongate and overlap their neighbors, but remain attached to the basement membrane by fine cytoplasmic stalks. The fetal epithelial cells do not markedly vary in size. Within the basal layer, which is the site of mitotic activity, many cells are large and binucleate, which suggests that basal cell fusions occur. It is believed that these cells eventually become surface cells. The luminal membrane develops its asymmetric structure (asymmetric unit membrane; AUM) following the appearance of AUM-bound vesicles in the apical cytoplasm, indicating that they are the source of the luminal AUM. The vesicles originate in the Golgi cisternae and are initially cup-shaped. The immature surface cells are characterized by microvilli of varying density on the luminal surface. The characteristic mature surface ridges appear to develop from rows of microvilli which fuse and link with other rows to form a reticular pattern. The process commences shortly after mid-gestation. By -15 to -10 days, the majority of cells have a well-developed surface pattern. Microvilli, however, remain prevalent in some small surface cells. These are believed to be the younger surface cells derived from pyriform intermediate cells, with the surface not fully differentiated. During the first 3 postnatal weeks, the basal cell population reaches the adult level and mitotic activity declines. The epithelium also acquires its adult morphology, whilst its functional maturity is indicated by the adult distribution and content of both glycogen, which is reduced, and alkaline phosphatase, which is increased. Rejection of cells occurs both antenatally and postnatally, and is probably a consequence of excess cell production.