The results of previous studies have suggested that periadolescent rats are differentially affected by catecholaminergic agents, when compared with younger or adult animals. Since dopamine-opiate interactions have been demonstrated in rats, the present study was conducted to evaluate the behavioral responsivity of periadolescent and adult rats following doses of morphine ranging from 1 to 10 mg/kg i.p. The responses measured included matrix crossings, rearing, grooming, auto-directed stereotyped behavior and gnawing, and were recorded continuously from 30 to 90 min post-injection. When indicated, scores on each measure were transformed to control for differences observed in the baseline (saline) groups. Analyses of all measures revealed a significant effect of morphine dose, although the shape of the dose-response curve differed for individual responses. In addition, those measures which might be considered locomotor responses (matrix crossing, rearing), as well as grooming, revealed significant main or interactive effects of age. In contrast, those measures characterized as stereotyped behaviors (auto-directed stereotyped behavior and gnawing) revealed no differential effect in periadolescent animals relative to adults. Since locomotor responses and stereotyped behavior have been suggested by previous research to be mediated by the mesolimbic and extrapyramidal dopamine systems, respectively, these results further support the previously suggested hypothesis of the delayed ontogeny of mesolimbic relative to extrapyramidal dopamine pathways.