The influence of centrally administered naltrexone, an opiate antagonist, on acquisition of intravenous cocaine self-administration behaviour in rats was examined. On five consecutive days, three hours per day, they could self-administer a cocaine solution (30 micrograms per infusion) through an indwelling cannula. Treatment consisted of daily injections of naltrexone (2 or 5 micrograms) or placebo into the lateral ventricle 30 minutes before testing. Naltrexone treatment dose dependently attenuated the rate of cocaine self-infusion. Both self-infusion rate and rate of responding on the reinforcement lever in the group treated with 5 micrograms naltrexone differed from placebo, whereas rate of responding on a dummy lever did not. These findings a) support the notion that opioid systems play a role in cocaine reinforcement, and b) suggest that naltrexone exerts its effect on cocaine reinforcement through action in the central nervous system.