The Mg2+-dependent ouabain insensitive-ATPase activity present in gill microsomal preparations from Dicentrarchus labrax is stimulated not only by Na+ but also by K=, NH4+ or Li+. These cations at 50-100 mM concentrations are similarly efficient to Na+ in stimulating the enzyme activity with similar Km values. Whatever cation stimulates the activity, the enzyme is poorly sensitive to ouabain and 100% inhibited by 1.5-2.5 mM ethacrynic acid. All activity vs cation concentration curves show a biphasic profile with activation following the Michaelis-Menten kinetics (Hill coefficient approximately 2). The absence of additivity when the enzyme is activated by binary mixtures of cations, each of which may act as competitive inhibitor of the other confirms the involvement of the same binding site for the monovalent cations.