This study aimed to determine whether exposure to stress during developmental period causes permanent behavioral and/or neurochemical alterations. Alterations of behavior were studied in young and aged rats which have been exposed to uncontrollable and unpredictable electric shocks on postnatal day(PND) 14 or PND 14 and 21. The concentrations of monoaminergic neurotransmitters were also measured to determine whether the behavioral alterations were accompanied by neurochemical changes. The results obtained are as follows: 1) The rate of increase in body weight was reduced at one day after exposure to the 1st series of shocks on PND14. However, these findings could not be observed after exposure to the 2nd series of shocks on PND 21. 2) Explorative activity decreased at one day after exposure to the 1st series of shocks on PND14. However this findings could not be observed after exposure to the 2nd series of shocks on PND 21. 3) At 100 days of age, there were little changes in the spontaneous locomotor activities measured for consecutive 23 hrs. However, there was positive correlation between the shock number showing the 1st helplessness during receiving the 1st series of shocks and the night time ambulatory activity of females, and was negative correlation between the shock number showing the 1st helplessness during receiving the 1st or 2nd series of shocks on PND 14 or 21 and the night time ambulatory activity of females. 4) At 360 ~ 390 days of age, night time ambulatory activity decreased in female rats which have been exposed to shocks on PND 14 and 21, but not in males. 5) In the aged female rats, the concentrations of 5-HT, dopamine and their metabolites were not different among groups. However, the ratio of 5-HIAA/5-HT increased in the frontal cortices of rats exposed to shocks on PND 14 and 21. These results demonstrate that the early experience of serious stress results in persistent alterations of behavior accompanying altered neurochemistry, and aging may unmask a subtle neuronal deficit causes by the early experience of serious stress.