The subcellular distribution of membrane-bound ATPases was compared among terrestrial plants, seagrasses and marine algae by cytochemical techniques. High ATPase activity was detected in the copiously invaginated plsma membrane that was characteristic of transfer cells but not in the tonoplast of epidermal cells in mature leaves of seagrasses. Magnesium- or Ca(2+)-dependent ATPase activity was induced together with the characteristics of transfer cells during the development of leaf tissues able to resist seawater. Northern hybridization revealed the effective induction of the synthesis of mRNA for plasma-membrane H(+)-ATPase during the development of leaves. Such high ATPase activity was not detected in the smooth plasma membranes of marine macro-algae but was found in the membranes of some cytoplasmic vesicles or microvacuoles, providing evidence of the excretion of salts by exocytosis. It appears, therefore, that two essentially different methods for excreting excess salts have developed separately in these two classes of marine plants. The evolution of mechanisms of salt tolerance in the plant kingdom is discussed in terms of the differential subcellular distribution of ATPase activity.