Muricidal behavior of the rat was induced by repeated s.c. administration of cadmium (Cd) for 15 weeks. The number of muricidal rats increased with increase in the duration of Cd administration in association with greater accumulation of Cd in the olfactory bulb than in any other region of the brain. The whole brain norepinephrine (NE) level of the rats which had been given Cd for 15 weeks was significantly greater than that of the control rats; neither dopamine (DA) nor serotonin (5-HT) level was changed. The Cd-induced muricide is discussed.