Abstract: Recent research indicates that alcohol use/abuse is often initiated during the adolescent period and that brain reinforcement pathways (e.g., the mesolimbic dopamine [DA] pathway) are undergoing developmental transition. Our research focuses on the effects of ethanol administration on neural mechanisms associated with addiction in preadolescent (postnatal day [PND] 25), adolescent (PND 35, PND 45), and young adult (PND 60) animals. Using conditioned place preference (CPP) testing, we have shown that adolescent animals are unique in their responses to ethanol. Since CPP has been associated with contextually conditioned incentive motivation, our results suggest that younger animals may be more vulnerable to addiction. The present data reveal that adolescent animals are neurochemically distinct in response to ethanol's effects. Using in vivo microdialysis within the nucleus accumbens septi (NAcc), we have determined the DAergic response across development. Results reveal that basal levels of DA transition during the adolescent period and differ from preadolescent or adult animals. Specifically, PND 45 animals exhibited significantly higher, and PND 25 significantly lower, basal DA levels than all other ages examined. Further, repeated exposure to ethanol elevated basal DA levels significantly regardless of age or dose. Basal 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio also differed as a function of age, with PND 35 and PND 60 animals demonstrating the highest ratios, and PND 45 animals producing the lowest baseline levels. Repeated ethanol exposure produced significant changes in basal ratios as a function of age. Interestingly, PND 45 animals exhibited no change in ratios with repeated exposure, while all other ages demonstrated a dose-dependent rise in DOPAC/DA ratios. These data indicate an age-dependent difference in the homeostatic alterations of mesolimbic systems in response to repeated ethanol treatment, an effect that may manifest itself as differences in behavioral responsivity and conditionability to the drug and the drug's effects.