Little is known about the physiological and behavioral effects of testosterone when co-administered with cocaine during adolescence. The present study aimed to determine whether exogenous testosterone administration differentially alters psychomotor responses to cocaine in adolescent and adult male rats. To this end, intact adolescent (30-days-old) and adult (60-day-old) male Fisher rats were pretreated with vehicle (sesame oil) or testosterone (5 or 10mg/kg) 45 min prior to saline or cocaine (20mg/kg) administration. Behavioral responses were monitored 1h after drug treatment, and serum testosterone levels were determined. Serum testosterone levels were affected by age: saline- and cocaine-treated adults in the vehicle groups had higher serum testosterone levels than adolescent rats, but after co-administration of testosterone the adolescent rats had higher serum testosterone levels than the adults. Pretreatment with testosterone affected baseline activity in adolescent rats: 5mg/kg of testosterone increased both rearing and ambulatory behaviors in saline-treated adolescent rats. After normalizing data to % saline, an interaction between hormone administration and cocaine-induced behavioral responses was observed; 5mg/kg of testosterone decreased both ambulatory and rearing behaviors among adolescents whereas 10mg/kg of testosterone decreased only rearing behaviors. Testosterone pretreatment did not alter cocaine-induced behavioral responses in adult rats. These findings suggest that adolescents are more sensitive than adults to an interaction between testosterone and cocaine, and, indirectly, suggest that androgen abuse may lessen cocaine-induced behavioral responses in younger cocaine users.