Male Wistar rats maintained on a thiamine deficient diet showed mouse-killing aggression (muricide). On the 30th of experimental feeding, the incidence of this muricide was about 70%. Intracerebroventricular histamine suppressed the muricide induced by thiamine deficiency in a dose-dependent manner. Histamine H1-receptor blocking agents such as diphenhydramine, promethazine and chlorpheniramine also showed muricidal suppression, but astemizole which lacks central effects did not show muricidal suppression. Mianserin and iprindole showed muricidal suppression, but metiamide i.p. did not. On the 20th day of experimental feeding, the incidence of this muricide was 45.5%. Histamine synthesis inhibitors such as brocresine or .alpha.-fluoromethylhistidine could not enhance the muricide on non-killer-rats, but really suppressed the thiamine deficient killer-rats. Evidently muricide induced by thiamine deficiency is not mediated by the central histaminergic system, but pharmacologically characterized by antidepressants, antihistamines and histamine synthesis inhibitors.