Prior work has shown important effects of the early life environment on development of adult stress response systems in both rats and humans. The present thesis is based on experiments that attempt to explore: 1) adolescent stress responding at hormonal and behavioural levels, and 2) the effects of repeated adolescent stressor exposure on adult stress responding (hormonal and behavioural) and levels of dopamine receptors expressed in prefrontal cortex, using both male and female rats. Defensive behaviours exhibited during stressor exposure and post-stress levels of circulating corticosterone were quantified as behavioural and hormonal measures of stress responding, respectively. In the first study, responses were compared among groups of adolescent rats exposed repeatedly to one of two different types of cat odour stressor stimuli (J-cloth coated in hair/dander or cat collar previously worn by a cat) or control stimuli, and long-term outcomes were examined in adulthood. Adolescent rats showed behavioural responses to both stressor stimuli, but behavioural inhibition was more consistent using repeated cat collar exposure, and this treatment resulted in long-term increases in anxiety-like behaviour in adulthood, whereas a stress-induced adolescent corticosterone elevation was observed only in the group that received exposure to the J-cloth stimuli. In the second study, adolescent and adult rats were compared directly using repeated exposure to the cat collar stressor or control stimuli. Adolescents were found to be more sensitive to the effects of the stressor stimuli, relative to adults. Finally, in the third study, repeated exposure to the J-cloth stressor or control stimuli was used, and stressor-exposed females showed elevated baseline corticosterone levels prior to the final exposure. Furthermore, stressor-exposed males and females showed lower levels of the D2 dopamine receptor in infralimbic and dorsopeduncular cortices of the prefrontal cortex in adulthood. In addition, these studies together provide evidence that sex differences in corticosterone levels emerge during the adolescent period. It may be concluded that adolescence should be considered a sensitive developmental timeframe for stress response programming.