In teleosts the insulin-like growth factor-I, a member of IGF system, play an important role in the control of growth and reproduction including metabolism, cell division, oogenesis, final oocyte maturation and secretory activity. This system is controlled by several hormones and metabolic status. Our previously in vitro data showed that, IGF-I have potency in resumption of meiotic arrest in the Persian sturgeon oocyte. In this study, serum IGF-I, thyroxine (T4) and cortisol levels were studied in two female brood stocks (caught from sea and from river during their up stream migration) during their late stage of sex maturation. Blood samples were taken from 6 individual in each brood stocks, and at three ovarian developmental stages based on their polarization index (IP). The first sampling was taken when PI was less than 0.7, the second when PI was more than 0.1 and the third when fish were ovulated. A heterologous radioimmunoassay system which used polyclonal anti-human IGF-I and human IGF-I as antigen, was used for IGF-I serum measurements. T4 and cortisol concentrations were also measured by radioimmunoassay using commercially available kits. Our results did not show significantly differences in IGF-I, T4 and cortisol levels between two brood stocks. However, in both brood stocks the mean IGF-I serum concentrations were higher in the second and third sampling stages than the first one (p<0.05). In the river brood stock, T4 serum level was significantly higher during ovulation than the second stage, but it was not observed in sea brood stock (p< 0.05). Cortisol serum levels had not showed significantly differences between three sampling stages in both brood stocks. These results could provide evidence for a possible role of IGF-I during final oocyte maturation of Persian sturgeon. The up regulation of serum IGF-I may have a role in the resumption of meiotic arrest during late stage of sex maturation. However, we could not found any relationship between IGF-I, T4
