The effect of morphine on ATPase of synaptic plasma membranes (SPM) and synaptic vesicles isolated from the mouse brain was studied. The activity of synaptic vesicle Mg++-dependent ATPase from mice rendered morphine tolerant and dependent by pellet implantation was 40% higher than that from placebo implanted mice. However, the activities of Mg++-dependent ATPase and Na+, K+ activated ATPase of SPM of tolerant and nontolerant mice were not significantly different. The activity of synaptic vesicular Mg++-dependet ATPase was dependent on the concentration of Mg++ but not of Ca++; maximum activity was obtained with 2 mM MgCl2. On the other hand, Mg++-dependent ATPase activity of SPM was dependent on both Mg++ and Ca++, activity being maximum using 2 mM MgCl2 and 10-5 M CaCl2. It is suggested that this stimulation of ATPase activity may alter synaptic transmission and may thus be involved in some aspects of morphine tolerance and dependence.