The similarity (or lack of) of mechanisms for ion and water transport across segments of the distal nephron of the various vertebrate classes is considered. Except for the reptilian distal nephron, the early distal nephrons from vertebrates of different classes appear to share certain morphological and functional characteristics. Among these are a relative impermeability of the tubule to water and the ability to preferentially reabsorb solute. This osmodilution of the luminal contents seems to be attributed to the presence of a sodium-chloride cotransport system located in the lumen membrane. The characteristics of solute and water transport of the late distal tubule are also considered. The available data suggest that there are striking similarities for the solute transport characteristics of the various vertebrate classes which have been studied. On the other hand, not all vertebrates appear to have developed the hydroosmotic response to antidiuretic hormone that has been observed in the mammals.