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Administration of the non-selective opioid receptor anagonists, naloxone and naltrexone, attenuate the rewarding effects of cocaine. The relative contributions of specific opioid receptor subtypes that underlie this effect have not been well characterized. Administration of 20.0 mg/kg cocaine resulted in a conditioned place preference. Pretreatment with 20.0 mg/kg but neither 10.0 nor 1.0 mg/kg of the selective μ₁-opioid receptor antagonist, naloxonazine, blocked cocaine-induced conditioned place preference. On the days in which rats received cocaine only, locomotor behavior was elevated. Pretreatment with the selective μ₁-opioid receptor antagonist, naloxonazine, regardless of dose, had no effect on cocaine-induced hyperlocomotion. These findings indicate that the rewarding effects of cocaine can be blocked solely by μ₁-opioid receptor antagonism and are consistent with the view that the locomotor and …