A fast Na+-exchange is shown to be present in isolated renal brush border membranes. The lower limit of the rate constant for this process, calculated from the 23Na-NMR spectrum is 580 sec-1. The actual exchange rate may be higher. A fast 7Li exchange is also shown to be present in the isolated membrane vesicles. The characteristic overshoot of the Na+ dependent D-glucose cotransport and Na+ H+ antiport can be demonstrated. The fact that neither treatment with papain, nor lowering of the temperature to 5°C affected the 23Na-NMR spectra obtained in the renal brush border membrane vesicles is consistent with the possibility that the fast Na+-exchange occurs through a channel mechanism. © 1985.