The action of some steroid drugs on aggressive behaviour was studied in mice and rats, using fighting isolated mice and muricide rats for this purpose. It was seen that hydrocortisone increased aggressiveness while hydroxydione decreased it, in both the above-mentioned tests. Desoxycorticosterone decreased the muricide reaction in rats, but failed to change the aggressive responses of isolated mice. In contrast, G-strophanthin decreased mice aggressiveness but exerted no effect on the rat muricide reaction. These findings are discussed in the context of the action of adrenal steroid hormones and other steroid drugs on various parts of the central nervous system.