Muricide in olfactory bulbectomized rats (OB rats) is readily inhibited by systemic administration of desipramine (DMI) or microinjection of DMI and noradrenaline (NA) into the medial amygdaloid nucleus. The present experiment investigated whether the muricide inhibition produced by these forms of drug treatment was mediated by alpha- or beta-noradrenergic receptors in the central nervous system. Muricide inhibition produced by systemic administration of DMI was antagonized by an alpha-blocker phenoxybenzamine but unaffected by a beta-blocker sotalol, although administration of these adrenergic blockers alone had no effect on muricide. Muricide inhibition induced by the microinjection of DMI and NA into the medial amygdaloid nucleus was similarly antagonized by pretreatment of phenoxybenzamine injected into the same site, but sotalol had no effect. Injection of phenoxybenzamine or sotalol alone into the medial amygdaloid nucleus did not elicit any changes in muricide. These findings suggest that mechanisms mediated by brain noradrenergic alpha-receptor play an important role in muricide inhibition by tricyclic antidepressants in rats and that the medial amygdaloid nucleus is an important site of action of these drugs.