Sexual maturity and levels of two main steroid hormones in gonads, 17-β estradiol (E2) and testosterone (T) relations which occur in female kutum Rutilus frisii kutum during spawning season from the river Sefid-Rood of the southern Caspian sea were studied using histological and radioimmunoassay. The study was carried out from February to May, 2008 using 105 fish specimen. The results of present study revealed that changes in plasma levels of gonadal steroids, (E2) and (T) were closely correlated to ovarian development and increased in GSI (p < 0.05). Gonadosomatic index (GSI) was increased in March and reached the highest value (29.47 ± 4.2) in April then decreased sharply in early May. The highest peak of plasma level of (T) and (E2) showed during spawning season, associated with the highest values of GSI. The results showed that levels of (E2) and (T) in female kutum at the stage IV of sexual maturity was significantly highest compared to immature gonads (ovary in stages II and III) (p < 0.01). Plasma estradiol (E2) and (T) levels increased in February, highest levels was observed in March and early April (105.6 ± 75.3 and 29.2 ± 96.6 ng/ml, respectively), and decreased in late April and in early May during the spawning season (p < 0.05).
