Based mainly on recent morphological and electrophysiological evidence, a proximal tubular cell model is discussed in which two parallel compartments are part of the intraepithelial fluid pathway. A number of observations are considered which suggest that there exists a functionally important extracellular pathway in addition to the transcellular route of ion movement. In contrast, presently available evidence suggests that in the distal tubular epithelium such low-resistance fluid paths between lumen and peritubular fluid are not functionally important. Several properties of distal tubular cells are described and incorporated into a cell model resynthesizing some of the events thought to be important in the transport of sodium, potassium and hydrogen ions.