Contextual stimuli associated with drug exposure can modulate various effects of drugs, but little is known about their role in relapse to drug seeking. Using a renewal procedure, the authors report that drug-associated contextual stimuli play a critical role in relapse to drug-seeking previously maintained by a heroin-cocaine mixture (speedball). Rats were trained to self-administer speedball, after which drug-reinforced behavior was extinguished over 20 days in the self-administration context or in a different context. On the test day, rats exposed to the drug-associated context, after extinction in a different context, reliably renewed drug seeking. The authors suggest that the renewal procedure can be used to study mechanisms underlying relapse to drug seeking elicited by drug-associated contextual stimuli.