Electrical measurements with microelectrodes have proven of great value in the investigation of renal ion transport mechanisms. By virtue of their fine tips (less than 1 micron in diameter) and because of the ease of recording electrical transients, they provide measurements with unparallelled space and time resolution. Early electrophysiological work was done largely in amphibians, because of their larger cell size, whereas studies in mammals were restricted to transepithelial electrical parameters, which provided no insight into individual membrane mechanisms. However, recently techniques have been developed to impale individual tubular cells of mammalian kidneys successfully, both in vivo and in vitro. Such experiments allow us to identify ion transport properties of individual cell membranes from the response of the electrical potential to fast pertubations of the luminal and/or peritubular fluid composition or to applied currents. The power of this approach was greatly increased by the development of ion-selective microelectrodes, allowing us to measure intracellular concentrations of Na+, K+, Cl-, Ca2+, and HCO3- or pH directly and to follow quantitatively their changes in response to different experimental maneuvers. In the present paper our knowledge of ion transport mechanisms of mammalian proximal tubular cell membranes is summarized, with emphasis on the transport of Na+, K+, HCO3-, and Cl-. With the exception of transcellular transport of Cl-, which is quantitatively less important, the major transport mechanisms of all other ions have been identified in the brush border and in the peritubular cell membrane. Emphasis is given to the description of HCO3- exit across the peritubular cell membrane, which has not thus far been studied with other than electrophysiological techniques. Microelectrode techniques will probably continue to provide new insight when regulatory phenomena are studied on the cellular level and individual conductance channels in renal cell membranes are identified with the newly developed patch-clamp technique.