The involvement of central α2-adrenoceptors in the development of muricide (mouse-killing behavior) in the olfactoly bulbectomized (OB) rats was strudied. The fractional turnover rate of noradrenaline (NA) in forebrain from OB muricidal rats was significantly lowered than that from sham rats. However, after yohimbine or idazoxan treatment, NA turnover was significantly increased in concert with inhibition of the development of muricide in OB rats. By radioligand binding studies with [3H] yohimbine, the maximum binding (Bmax) of [3H] yohimbine to cerebral cortical membrane from OB rats was significantly higher than that from sham rats without change of affinity for agonists and antagonists(Ki). With these results it was suggested that muricidal behavior of OB rats has a close relevance to the hyperactivity of central α2-adrenoceptor system.