Studies were performed on the effects of the activation of the kinin-forming system on the action of chlorpromazine in rats and mice. It was found that in animals with CNS kinin enzymes previously activated with kallikrein, or in animals which had been given bradykinin, the psychodepressive activity of chlorpromazine became markedly higher. The administration of Trasylol® brought about a decrease of the postkallikrein activation of kinin-generating enzymes and abolished the potentiating kallikrein effect on the action of chlorpromazine. Under multiple pathological conditions which are followed by increased kinin levels, an altered potency of drugs acting on CNS should be taken into consideration.