This study examined whether there are differences in the behavioral and neurochemical effects of cocaine in periadolescent rats compared to adult rats. Periadolescent (postnatal days 28-35) and adult rats were injected with cocaine or vehicle for 7 days. Ten days later (day 17), rats either were challenged with cocaine, or dopamine transporter and receptor and serotonin transporter binding were examined. Adult rats became sensitized to the locomotor-activating effects of cocaine and there were increases in dopamine transporter density in the caudate putamen compared to vehicle-treated adult rats. In addition, serotonin transporter densities were increased in the ventromedial caudate putamen, nucleus accumbens shell, and the olfactory tubercle in cocaine-treated adult rats compared to vehicle-treated adult rats. In contrast, periadolescent rats did not show sensitization to cocaine and there was no effect of cocaine on either dopamine or serotonin transporter densities. These findings suggest that there are different neurochemical and behavioral adaptations to repeated cocaine administration in periadolescent versus adult rats.