Snake (Thamnophis spp.) proximal renal tubules were perfused and bathed in vitro either with medium containing sodium or with medium in which the sodium was replaced with choline. Net fluid absorption was measured by changes in volume marker concentration, and cell volumes and cell membrane surface areas were measured by ultrastructural morphometric methods. Net fluid absorption did not differ significantly in the presence or absence of sodium. However, during the 20-25-min perfusion in the absence of sodium, significant morphological changes took place. The volume of the cells, doubled and the volume of the intercellular spaces nearly quintupled. The areas of the lateral and apical cell membranes approximately doubled, but their surface densities remained essentially constant. Therefore the larger cells in the absence of sodium had proportionally enlarged surface areas, so that the volume-to-surface area ratio remained constant. These morphological changes occurred concomitantly with the maintenance of net fluid absorption and might play a permissive role in such maintenance in the absence of sodium.