The basic cellular transport mechanisms for the transepithelial uptake and elimination of sodium and chloride ions are briefly reviewed. These frequently involve processes defined as secondary active mechanisms, and the problems that this creates for their identification in epithelial systems are emphasised. It is suggested that despite the wide range of epithelial tissues involved in body fluid ion regulation, current evidence indicates that a relatively limited group of these secondary active carriers is involved in the transport process, and the basis for this evolutionary conservatism is discussed, together with certain of its implications. Finally, the interactions of the transport mechanisms with metabolic and acid–base parameters are contrasted in ion-uptake and ion-excretion situations, and the possible roles of carbonic anhydrase compared.