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Synaptic plasma membranes isolated from bovine brain exhibited a low and high affinity calcium (Ca++) + magnesium (Mg++)-dependent ATPase as evidenced by kinetic constants for ATP. One activity that hydrolyzed ATP maximally at pH 7.4 and 7.8 exhibited an eight-fold higher affinity when compared to the second or lower affinity activity that hydrolyzed ATP maximally at pH 7.0. Only slight inhibition was observed in the presence of rotenone and oligomycin. Although both activities were observed to be trifluoperazine sensitive, they differed significantly with regard to other parameters. Thermal denaturization studies indicated that the high affinity activity was stable for 2 min at 45C aft 50% of the activity was lost at 2.5 min. Although kinetic data are consistent with other findings that indicate the presence of different kinetic conformations of a single synaptic membrane protein, we can not rule out the presence of two different proteins maximally operational at two different ATP concentrations.