In two experiments we have found and replicated the observation that intraseptal muscimol profoundly facilitates muricide. It also increases irritability (response to handling). These effects are specific to aggressive behaviors in that the drug affects neither activity nor chocolate chip acceptance. The effects of the GABa synthesis inhibitor thiosemicarbazide depend upon the site of injection within the septum; in more anterior loci the drug produces the expected increase in muricide latency; in more posterior sites it produces an anomalous facilitation of muricide. The serotonergic agents quipazine and metergoline have no significant effect when injected into any of these sites. These results suggest that the septal neurons mediatingthe muricide-inhibitory effect of electrical stimulation  are subject to local, GABAergic, control. Inhibition of these neurons by muscimol produces a net disinhibition. of muricide.