The aim of this study was to investigate the effects of captopril injection into Nucleus Accumbens (NAC) on morphine self-administration. Male Wistar rats were first trained to receive small pellets of food by pressing the active lever in self-administration apparatus. The animals, divided into 3 groups (saline, morphine, and captopril), were placed in self-administration apparatus and allowed to self-administer morphine (5 mg/infusion in all test groups) or saline (saline group) for 5 consecutive days, for 2 h/session. Captopril (30 μg) was injected into NAC in the captopril group before each session. In morphine group, the number of active lever pressing was significantly higher than that of the passive one during all 5 days and was also significantly higher than the saline group. In captopril group, there was no significant difference between the number of active and passive lever pressings. However, the number of active lever pressing was significantly lower than morphine group. The results highlight the interaction between captopril and opioid system in NAC.