The kinetics of the hydrolysis of ATP by a rat-brain lipoprotein fraction have been investigated. The activity is greatly increased in the presence of Na+ and K+, but reaches a maximum value and then suffers a diminution in the presence of an excess of either ion. Kinetic evidence indicates that these ions combine with the enzyme protein according to Michaelis-Menten kinetics, that 2 Na+ are involved in the activation, and that the interaction of Na+ and K+ at each site is of a competitive nature. Inhibition of the (Na+K+)-stimulated ATPase by ouabain, appears to result from a competition between ouabain and both K+ and Na+ while addition of oligomycin leads to kinetics which suggest competition of oligomycin with Na+. NH4+ may substitute for K+ and, when present in excess, may also give rise to inhibitory effects.