Volume and osmolarity of urine produced by kidneys of reptiles, birds, and mammals depend on anatomic relationships among nephrons, epithelial permeability to water controlled by antidiuretic hormone, and, for reptiles and birds, probably on volume flow rate through collecting ducts and excretion of uric acid. Urine volume and volume flow rate through collecting ducts in reptiles and birds depend on number of filtering nephrons controlled by antidiuretic hormone. Mammalian nephrons do not filter intermittently but control of nephron filtration rates in all three vertebrate classes may have important similarities and differences. Uric acid excretion by birds and many reptiles permits excretion of inorganic cations in excess of amounts permitted by osmolarity of urine. This process may require tubular absorption of water without sodium. Such absorption, which has been found in reptilian proximal tubules, may be very important for osmoregulation in all birds and uricotelic reptiles and may provide insight into the mechanism of fluid absorption in mammals. Urea excretion in mammals may be important for enhancing concentrating ability. Much more must be learned about these processes, but similarities and differences among them in the three vertebrate classes may help illuminate details of each.