In plasma membranes of the fish brain, we found a Cl – stimulated Mg 2+ ATPase whose “basal” Mg 2+ ATPase activity was stimulated by Cl – -ions (Cl − ATPase) . The properties of this enzyme differed significantly from those of the Cl – ATPase (Cl − pump) in the mammalian (rat) brain . In particular, the two enzymes differed in the pH optimum, ä å of the ATP hydrolysis, and sensitivity to inhibitors . In addition, Cl – stimulated Mg 2+ ATPase of the fish brain was regulated by γ aminobutiric acid (GABA), the mediator that, via GABA receptors, controls the Cl − transport through the postsynaptic membrane [4, 5]. Thus, at pH 8.5 of the incubation medium, this mediator activated the basal Mg 2+ ATPase activity and inhibited the Cl – ATPase activity . Conversely, at a physiological (pH 7.4) of the incubation medium, when the Cl – ATPase activity was not detectable, whereas the basal Mg 2+ ATPase activity was increased, GABA inhibited the latter and induced the Cl – ATPase activity . The GABA effect on the enzyme was eliminated after addition of picrotoxin, which blocked GABA receptors [4, 6]. These data suggest that the mediator of the suppressing receptors affects the Cl – stimulated Mg 2+ -ATPase of the fish brain in a receptor-dependent manner.