The authors studied the effect of Rb, Li and Cs on the ATPase system and c[cyclic]AMP protein kinase in [rat] brain. They demonstrated that Rb could replace K+ in the Na+K+-ATPase system; Li and Ca had no effect on this enzyme activity in the absence of K+. K+-dependent ATPase was activated by even low Rb concentrations; Li and Cs inhibited it. None affected c-AMP protein kinase.