Previous experimental investigations of embryonic kidney development with in vitro organ-culture techniques have been limited by the short duration of graft viability and lack of detailed anatomic development. In a modified application of the chick chorioallantoic-membrane (CAM) grafting technique, we have transplanted 11-day-old undifferentiated embryonic murine kidney rudiments and studied their differentiation and early morphogenesis. Surviving grafts, maintained for 5-10 days on the CAM, demonstrated well-developed convoluted secretory tubules and highly branched collecting ducts. Definitive glomeruli were also identified in these grafts by the presence of efferent tubules, visceral and parietal epithelium, capillary tuft, and Bowman's space. In vitro branching of the ureteric bud preceded formation of definitive tubules. Sections of the CAM grafts appeared histologically comparable to the 14-15-day in vivo embryonic metanephros. Grafts of 10-day-old rudiments showed minimal kidney development; fewer grafts survived, fewer tubules were formed, and fewer glomeruli developed. Our results indicate that the CAM is an efficient site for in vitro murine kidney development with both vascularization of the explant and extensive maturation, including the appearance of primitive nephron units.