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The effect of vanadium chloride on rat brain synaptosomal adenosinetriphosphatase (ATPase) activities was determined in vitro and in rats treated at 1 mg/kg.d ip and 10 mg/kg.d po for 10 d. Additional experiments were conducted to determine the effect of vanadium chloride on binding of [3H] ouabain and 45Ca to rat brain synaptosomes. Na+ + K+ - and Ca2+-ATPase activities were inhibited significantly in a concentration-dependent manner by V in vitro. Mg2+ -ATPase inhibition was neither dose-dependent nor significant except at 10(-5) M. Na+ + K+ -ATPase inhibition by V was more pronounced than that of other ATPases studied. Vanadium inhibited [3H] ouabain binding to synaptosomes by 90% at 10(-3) M; the inhibition was concentration-dependent. Binding of 45Ca was inhibited 50% at 10(-4) M; but concentration-dependent inhibition was not evident. Rats treated with vanadium chloride neither became myotonic nor showed any changes in ATPase activities or binding of [3H] ouabain and 45Ca to brain synaptosomes. Lineweaver-Burke plots of the in vitro inhibition of Na+ + K+ -ATPase and [3H] ouabain binding revealed that (1) Na+ + K+ -ATPase activation by ATP was inhibited by V with an increase in Km and a decrease in Vmax; (2) Na+ activation was inhibited noncompetitively by V, as evidenced by a decrease in Vmax and no change in Km; (3)K+ activation was inhibited by V with a decrease in both Vmax and Km; (4) noncompetitive inhibition of Mg2+ -ATPase by V was observed; and (5) the kinetic behavior of [3H] ouabain binding inhibition by V with respect to ATP and Na+ activation was mixed and noncompetitive, respectively. These results suggest that V is a potent inhibitor of Na+ + K+ -ATPase activity in rat brain synaptosomes.