The comparative physiology of the renal proximal tubule (PT) is reviewed in the context of vertebrate evolution and vertebrate strategies of salt and water balance. Though extrarenal machanisms of salt and water balance contribute importantly to extracellular fluid (ECF) homeostasis in the lower vertebrates, the kidney acquires these functions with evolutionary progress and becomes the dominant organ of ECF homeostasis in mammals. In acquiring the major responsability over the ECF compartment the kidney favored filtration-reabsorption as the preferred mechanism for the rapid turnover of ECF with the advantage of providing quick renal regulatory responses. In spite of this specialization the structure and function of the PT do not appear to have undergone major evolutionary changes. In present-day vertebrates the PT remains as an immensely diverse transport epithelium with impressive capacities for both reabsorptive and secretory work, as exemplified by the mammalian PT with mostly reabsorptive functions and the PT of aglomerular kidneys with mostly secretory functions. The recent evidence for NaCl and fluid secretion in the PT of, unexpectedly, glomerular kidneys is consistant with the functional diversity and the conservative nature of evolution in the case of the PT.