Effect of chronic treatment of standardized aqueous extract of Moringa oleifera (MO) root (100, 200, 300, 350, 400, 450 mg/kg; po) on penicillin (PCN) induced convulsion, locomotor behaviour, brain serotonin (5-HTT), dopamine (DA) and norepinephrine (NE) level was studied in Holtzman strain adult albino rats. The result revealed that pretreatment with MO inhibited PCN-induced seizure and markedly reduced locomotor activity. Chronic treatment with MO significantly increased the 5-HT and decreased the DA level in cerebral cortex (CC), midbrain (MB), caudate nucleus (CN) and cerebellum (CB). NE level was significantly decreased in CC but no appreciable change was observed in MB, CB and CN. Thus the central inhibitory effect of MO is discussed in the light of the disturbed balance between 5-HT, DA and NE.