Both drug-related cues and drug priming are the main factors that induce relapse of drug addiction. Previous research has reported that blockade of the muscarinic receptors could significantly depress addictive behavior, suggesting that the muscarinic receptors might be involved in drug use and relapse behavior. The nucleus accumbens (NAc), especially the shell of the NAc, where the muscarinic receptors are expressed, is critical for craving and relapse. This study investigated the effects of microinfusion of the muscarinic receptor antagonist scopolamine into the NAc shell on context- and morphine-induced expression of behavioral sensitization. Behavioral sensitization was established by exposure to 5 mg/kg morphine once daily for five consecutive days. Expression of behavioral sensitization was induced by saline challenge or 5 mg/kg morphine challenge. The results showed that: (a) the muscarinic receptor antagonist scopolamine (10.8 μg/rat) microinjected into the NAc shell blocked expression of conditional sensitization; (b) acetylcholinesterase inhibitor huperzine-A (0.5 and 0.1 μg/rat), but not scopolamine (10.8 μg/rat), microinjected into the NAc shell blocked morphine-induced expression of sensitization; and (c) pre-infusion of scopolamine (10.8 μg/rat) reversed the inhibitory effect of huperzine-A (0.5 μg/rat) on morphine-induced sensitization. Our findings suggest that muscarinic receptors in the NAc shell play different roles in context-induced and morphine-challenged expression of behavioral sensitization.