Non-contingent, 'priming' IV drug injections led to reinstatement of heroin-reinforced responding after a period of extinction. Rats implanted with IV catheters were trained to self-administer heroin (100 micrograms/kg/infusion diacetylmorphine HCl) and were given test sessions consisting of a period of self-administration followed by extinction conditions. 'Priming' infusions of heroin and other drugs were presented during extinction and lever pressing following the injection was observed. Priming injections of 50, 100 and 200 micrograms/kg heroin effectively restored responding after a period of extinction. Morphine and, to a lesser extent, amphetamine and apomorphine also reinstated heroin-reinforced responding whereas cocaine and clonidine did not. These results may help our understanding of 'relapse of drug-taking' behavior.