Objective: Immunohistochemical localization and immunoreactivity of insulin-like growth factor-I (IGF-I) were investigated during the ovarian developmental stages in Persian sturgeon. In addition, the effects of growth hormone and thyroxine were investigated on IGF-I immunoreactivity in vitro. Materials and Methods: Ovarian samples were taken from two brood stocks (caught from the sea and from a river) during their reproductive migration and at three different developmental stages based on their polarization index (PI). The effects of two hormones on IGF-I immunoreactivity were studied at two ovarian developmental stages (PI > 0.1 and PI < 0.07). In both experiments the immunoperoxidase method was performed in order to detect immunohistochemical localization and immunoreactivity of IGF-I. Results: Immunohistochemical localizations of IGFI were detected in the follicular layers. There was no significant immunoreactivity difference between the two brood stocks, however in the river brood stock IGF-I immunoreactivity was more intense in the ovaries with PI < 0.07 than of PI > 0.1 (p < 0.05). Growth hormone (10 ng/ml) increased IGF-I immunoreactivity in ovarian samples from the river brood stock when their PI was less than 0.07, however thyroxine had not such effect (p < 0.05). Conclusion: Our results showed that IGF-I is present in the ovaries of Persian sturgeon and its reactivity is different among their gonadal development stages. This may support a role for IGF-I during reproductive physiology in female brood stocks of the Persian sturgeon. Moreover, growth hormone is a potential hormone to increase IGF-I immunoreactivity in the ovaries of this species.