In rats trained to self-administer heroin or cocaine intravenously, non-contingent priming injections of heroin or cocaine, respectively, reinstated responding after a period of extinction. In rats similarly trained to self-administer heroin or cocaine intravenously, morphine sulphate was applied centrally to sites in the ventral tegmental area (VTA), the periventricular grey (PVG) and the caudate nucleus following a period of extinction. Self-administration behavior was reinstated by morphine application to sites in the region of the VTA, but not to other sites, in both heroin and cocaine-trained animals. This priming effect of morphine was blocked or attenuated by prior administration of naltrexone, given intraperitoneally. Morphine in the VTA is known to activate mesolimbic dopamine neurons, suggesting that dopamine activity in this system may underlie the priming effects of both opiates and stimulants. Furthermore, the fact that the mesolimbic system is implicated in the positive motivational actions of both drug groups, suggests that morphine reinstates drug-taking behavior in these animals by activating appetitive motivational systems of the brain.