In addition to the well established action of PTH in proximal tubules and of AVP in collecting tubules, polypeptide hormones were recently shown to regulate transport properties in other tubular portions. Although still scarce, such physiological studies using isolated perfused tubules demonstrated hormonal effects in those nephron segments observed to contain responsive adenylate cyclase and not in the others. Moreover, the same effects were elicited by applying exogenous cAMP or cAMP derivatives. There is, therefore, good evidence that hormone-dependent adenylate cyclase is involved in the cell mechanisms through which many hormones regulate tubular functions. The effects obtained varied depending on the segment of tubule used. It is not yet established whether the nature of the hormonal effect induced via cAMP is entirely specified by the responding cell types or is also specified by the hormone itself. Further studies are needed to clarify this important problem, as well as many other as yet unsolved questions. There is obviously much more to learn about the hormonal regulation of tubular cell functions by using appropriate biochemical and physiological micromethods.