Background and Objective: The effect of opioids on the reproductive system has been studied as an attractive research field. In this study, the effect of morphine as an analgesic opioid was evaluated on the ovarian polycystic induction in the rat. Materials & Methods: In this study, 24 female virgin Wistar rats weighing 200-250 g were injected with morphine (1-10 mg/kg/day) for 9 days. Control group received normal saline (1 ml/kg, i.p.) throughout the experimental period. After completion of injection period, all animals underwent surgery to collect the ovaries. After biometrical measurements, the ovaries specimens were fixed in the formalin to use in future study. Results: The ovaries from morphine-injected rats had thick-wall follicular cysts whilst the control samples showed the Graafian follicles. In addition, the higher doses of morphine (10 mg/kg) led to much thicker wall cysts than the lower doses (1 mg/kg). However, the experimental samples did not show a significant difference versus control in view of their size. Conclusion: The enhanced irregularity in the opioid system activity has been reported in polycystic ovary syndrome (PCOS). Based on these findings, the induction of connective disturbance between the opioid and the reproductive systems due to chronic use of the morphine may crucially play a role in ovarian polycystic induction in the rat.