Experimental evidence has shown that an intermittent food delivery schedule interacts with the pharmacological properties of a variety of drugs to produce significant increases in self-injection rates. This self-infusion drug seeking behaviour is further strengthened when body weight reduction is introduced. This experiment studies the effect of the opiate antagonist naloxone upon a strongly established response pattern for heroin self-administration, using naive rats at 80% body weight which were made dependent upon the drug by the schedule-induced self-injection procedure. Results show a clear reversal in responding following naloxone treatment, suggesting that once an interaction between schedule-induced behaviour and a drug is established, blockage of the reinforcing effects of the drug extinguishes the behaviour altogether.