Mammals and birds are the only vertebrates known to produce a concentrated urine by means of renal medullary countercurrent systems. These two countercurrent systems, however, exhibit functional and anatomical differences which appear to be related to the fact that mammals are ureotelic while birds are uricotelic. In mammalian kidneys, urea accumulation in the medulla plays an important role in the concentrating mechanism. In bird kidneys, there is no accumulation of urea in the medulla. The mammalian renal medulla is surrounded by a muscular, funnel–shaped pelvic wall, leaving an elaborate urinary space between the renal medulla and the inside of the pelvic wall, while the bird renal medulla is surrounded by tight sheets of connective tissue leaving no space for the urine to contact the renal medulla. The mammalian renal pelvis makes it possible for urine to contact the epithelial covering of the inner and outer medulla, and the peristaltic contractions of the muscular pelvic wall exerts a rhythmic pumping action on the renal papilla. The functional significance of these two aspects of the renal pelvis have in recent years become the focus of attention by some renal physiologists.