In the rabbit nephron, the luminal membrane surface area of the proximal convoluted tubule (PCT) is more than twice that of the proximal straight tubule (PST). What seemed to be an increase in histological specialization in solute and water transport is curiously reflected by a lower transepithelial water permeability per unit of apical membrane area in PCT than in PST. To evaluate what change in luminal membrane water permeability corresponds to this morphological difference, the osmotic permeabilities (Pf) of brush-border membrane vesicles isolated from PCT and PST of rabbit kidney were compared. D-Glucose uptake rates indicated proper separation of two populations of vesicles. Vesicle size measured by quasi-elastic light scattering was 123 +/- 7 nm and 125 +/- 6 nm for vesicles isolated from PCT and PST, respectively. Pf obtained by stop-flow light scattering techniques was of 106 +/- 6 microns/s in PCT vesicles and 191 +/- 7 microns/s in PST vesicles (T = 26 degrees C). In the presence of the sulfhydryl reagent HgCl2, the water permeabilities of both types of membrane dropped to comparable values. These data, which show an 80% increase in apical water permeability along the length of the proximal tubule, suggest that the number of proteic water channels per unit of membrane area is greater in PST than in PCT.