The present study investigates the effect of long term treatment (0.06,0.3 and 0.6g/kg b. wt., orally) with monosodium glutamate (MSG) on open field test and some biochemical parameters such as cortisol, serotonin (5-HT) and monoamine oxidase activity (MAO) of post weaned albino rats for 3 months. Open field test showed a significant decrease in animal freezing and marked increase in horizontal movement expressed by number of crossed squares as well as vertical movement represented by number of rearing. In addition, MSG treated groups showed marked increase of cortisol level in serum, as well as serotonin (5-HT) level in frontal cortex. Moreover, significant decrement in monoamine oxidase (MAO) activity in frontal cortex had been observed. Results provide evidences that post weaned rats chronically exposed to monosodium glutamate which used widely as food additives showed decrease in anxiety and increase in hyperactivity.