Progress curves of the enzymatic reactions show that ATPases of bulk isolated glial cells, perikarya and synaptosomes exhibit hysteretic change. Initial velocities of enzyme activities were therefore obtained according to the equation valid for the hysteretic model. The (Na+, K+)-ATPase activities of the same brain fractions were measured before or after NaI treatment. Only glial and synaptosomal enzyme could be adequately extracted by using this procedure. Attempts to purify the (Na+, K+)-ATPase from brain perikarya by NaI extraction were unsuccessful. In order to determine the effect of the K+ ions on enzymic physiological efficiency (phys. eff.; i.e., the ratio Vmax/Kmapp) the variation of (Na+, K+)-ATPase activities from each brain fraction was measured as a function of Mg.ATP2- concentration in the presence of 5 and 20 mM K+ ions. High K+ ion concentrations (20 mM) increased the physiological efficiency of glial enzyme and decreased the same kinetic parameter in neuronal (perikaryal as well as synaptosomal) enzyme preparations. Results are discussed in relation to a possible distribution of distinct enzyme in different brain cell populations as well as a possible role of glial cells in an active regulation of K+ ion extracellular fluid in the CNS.