The purpose of this study was to investigate the behavioral effects in young rats of subcutaneous injection of MSG in newborn rats. The results revealed that MSG exposure during the postnatal period increased the total distance traveled, number of fecal pellets, urine occurrence and number of animals' vocalization of young rats evaluated in the activity monitor. Furthermore, MSG-treated rats showed an increase in the time spent immobile during the FST and the freezing reaction in the contextual fear conditioning. In addition, we demonstrated an increase in the [3H]5-HT uptake in cerebral cortices, and higher levels of ACTH and corticosterone in the serum of MSG-treated rats