When rats were maintained on a thiamine-deficient diet for 30 days, about 70% of the animals showed a mouse-killing response (muricide). The thiamine-deficient killer-rats do not eat but merely killed the mice. Once this abnormal behavior appeared, the response remained, and could not be suppressed by the administration of thiamine hydrochloride plus thiamine-supplemented diet, regardless of a return to normal feeding, growth and heart rate. Drugs that activate the central serotonergic and noradrenergic systems have suppressive effects on it. Conversely, among the various depletions of brain monoamines, only depletion of serotonin by the drug p-chlorophenylalanine significantly increased the incidence of muricide. Antihistaminergic drugs were potently effective, but atropine, an anticholinergic drug, were ineffective. Various antidepressants and electroconvulsive shock treatment also suppressed muricide to various degrees. Thus, it is expected that the muricide induced by thiamine deficiency may be useful as an animal model of depression, although the usefulness of this abnormal behavior as a working model of depression or for screening new antidepressants remains to be evaluated.