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Objective: Immunohistochemical localization and immunoreactivity of insulin-like growthfactor-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-Iimmunoreactivity in vitro.Materials and Methods: Ovarian samples were taken from two brood stocks (caughtfrom the sea and from a river) during their reproductive migration and at three different developmentalstages based on their polarization index (PI). The effects of two hormones onIGF-I immunoreactivity were studied at two ovarian developmental stages (PI > 0.1 andPI < 0.07). In both experiments the immunoperoxidase method was performed in order todetect 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 ovarieswith PI < 0.07 than of PI > 0.1 (p < 0.05). Growth hormone (10 ng/ml) increased IGF-I immunoreactivityin ovarian samples from the river brood stock when their PI was less than0.07, however thyroxine had not such effect (p < 0.05).Conclusion: Our results showed that IGF-I is present in the ovaries of Persian sturgeonand its reactivity is different among their gonadal development stages. This may supporta 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 immunoreactivityin the ovaries of this species.