The functional properties of the proximal tubule in the salamander Ambystoma tigrinum have been well characterized but its ultrastructure has not been examined. We therefore analyzed the qualitative and quantitative ultrastructure of the proximal tubule in this species as a basis for structure-function correlations. In addition, a comparative study between species was performed between Ambystoma and Necturus. In Ambystoma the basal cell membrane but not the lateral cell membrane has a highly elaborate organization and is greatly amplified at the basal cell surface. Therefore, the bulk of the basolateral membrane does not face the lateral intercellular space but faces a basal extracellular labyrinth immediately adjacent to the peritubular space. We suggest that this intraepithelial compartment may serve as a site for solute-solvent interactions. The morphometric comparative analysis provides quantitative estimates of tubule dimensions, volume of cells and extracellular channels, areas of luminal, lateral, and basal cell membranes as well as averaged dimensions of the lateral intercellular spaces. Structure-function correlations show that when certain functional parameters are normalized on the basis of ultrastructural rather than epithelial dimensions the interspecies variability decreases.