Water diffusion in the in vitro microperfused rabbit proximal convoluted tubule was measured to assess the transepithelial pathway for water movement and the physical characteristics of the water permeation pathway. The measured diffusive water permeability (PDW(MEASURED] was high, 5 X 10(-3) cm/s, suggesting a transcellular transport route. Simultaneously measured n-butanol permeability (PDNB(MEASURED], an index of the cytoplasmic resistance to water diffusion, was 3.6 X 10(-3) cm/s. PDW(MEASURED], PDNB(MEASURED) and the free solution diffusion coefficients for water and n-butanol were used to derive a minimum membrane PDW (PDW(MEMBRANE] of 12.5 X 10(-3) cm/s. These data suggest that at least 60% of the transepithelial resistance to water diffusion resides in the cell cytoplasm. Measurement of the temperature dependence of PDW(MEASURED) and PDNB(MEASURED) gave an apparent activation energy of PDW(MEMBRANE) that was constant between 20 and 40 degrees C at about 4.3 kcal/mol. The organic mercurial sulfhydryl reagent p-chloromercuribenzene sulfonate, which has been shown to reduce diffusive water permeability in the red cell membrane by 50%, reduced PDW(MEMBRANE) by 54% without affecting PDNB(MEASURED). These last two independent lines of evidence are consistent with water diffusion through small, aqueous, protein-bounded channels.