The distribution of exogenously administered Li+, Rb+ and Cs+ in distinct mouse brain regions was studied as a function of time subsequent termination of a short-term daily treatment with these alkali metal salts. The resulting distribution profiles were compared with that obtained from the corresponding endogenous alkali metals. Endogenous Rb+ and Cs+ were readily measurable in all 6 brain regions studied compared to traces of measurable Li+. The Rb+ concentration was greater than that of Cs+. Short-term treatment with equal doses of the alkali metals studied showed greater brain accumulation of Rb+ and Cs+ than Li+ with a prolonged brain regions preference for Cs+ storages as a function of time. Duration of ethanol-mediated narcosis was reduced from saline controls by pretreatment with RbCl or CsCl as contrasted with prolongation by LiCl. The narcotic dosage of ethanol used reduced endogeneous Li+ and increased Cs+ levels in the cerebellum. This massive ethanol dosage exerted little effect on the distribution of exogenously administered Cs+ with exception of the striatum which continued to show a high content of Cs+. This may have contributed to partial antagonism of ethanol-depressant action and to restoring of motor function, i.e., rapid regaining of righting reflex. The results showed that Cs+ possessed longer biological life time in the brain than Rb+ or Li+ which may be of therapeutic value, i.e., in the use of Cs salts in treatment of brain tumors and in conjunction with psychoactive agents provided the respective chemotherapeutic and antidepressant properties of CsCl have been established.