Basal horizontal activity (A.) was determined in periadolescent and adult male and female rats for 2 h during initial exposure to the locomotor activity chambers. Periadolescent and adult rats were 29 and 64 days old, respectively. Beginning the next day, horizontal activity was assessed (2 h) in males (B.) and females (C.) immediately following the administration of saline or increasing doses of morphine. Dose-response data are normalized as a percentage of within-subject baseline activity. Each data point represents the mean ± S.E.M. (males n=7–8; females n=12). #Indicates significantly different from adults, p<0.05. *Indicates a significant difference from saline, p<0.05. @Indicates a significant gender effect, P<0.05.