Na+ uptake was measured in brush border membrane isolated from rat kidney cortex. In the presence of 100 mM NaCl gradient, Na+ equilibrated across the membranes within 30 minutes, but uptake was linear only for the first 10 seconds. Total uptake could be analysed in terms of a diffusion component and a saturable component (Kt = 5 mM, Jmax = 0.62 pmol/microgram prot/s). Uptake at 1 mM NaCl was found to be inhibited at 60% by amiloride; stimulation of uptake by pH gradient (inside greater than outside) supports the functioning of a Na+-H+ antiport. Uptake at 100 mM NaCl was insensitive to amiloride in the absence of pH gradient. The influence of anions on Na+ uptake was in agreement with known permeabilities for generation of electrical potentials (Thiocyanate greater than chloride greater than gluconate) and was observed at 100 mM Na+ but not at 40 microM. These results suggest that rat renal brush border membrane vesicles are leaky towards Na+ and present a permeability coefficient for Na+ much higher than what is expected from the in vivo conditions.