SUMMARYUptake of NaCI by amphibian tight epithelia, such as skin, urinary bladder and collecting duct, requires considerable thermodynamic work. By calculation it is demonstrated that NaCi absorption from dilute external solutions ([NaCl] 1 mM) demands more energy than can be provided by the Na+-K+-ATPase alone. Thus, in addition to the Na+ pump, another transport ATPase must be involved. Previously, we have suggested that the other transport ATPase is an apical proton pump in mitochondria-rich (MR) cells. By driving an apical Cl-HCO3-exchange, a rheogenic H' pump would energize entrance of Cl-across the apical membrane. Since Cl-channels are present in the basolateral membrane, the net result would be a transcellular active rheogenic uptake of Cl-, and secretion of H' and HCO3-. At higher external concentrations, the Cl-uptake by MR cells is driven by the Na+ pump-generated transepithelial electrical potential difference, which also requires the presence of apical Cl-channels. Electrophysiological methods have been developed by which we have been able to study the polarity of single MR cells and identify apical and basolateral transport systems. We have verified the existence of rheogenic H' pumps in the apical membrane and Clchannels in both membranes.