Serum levels of 17β-oestradiol (17β-E₂.), testercon? (T) and17α-hydroxy-20β-dihydropogesterone (17α20βP) were measeured in female rainbowtrouts (Salmo gairdneri) during their first ovulation. 17β-E₂ fell from 18.1 ng/ml15 days to 2ng/ml 3 to 6 days prior to ovulation, and it was 0.9ng/ml duringovulation. Testosterone reached to a peak 1evel of 143. 3ng/ml ct 9 days prior to ovula-tion, then the level fell to 24. 3ng/ml during ovulation. At 15 days before ovulation,17α 20β P levels were either nondetectable or extremely low, but it rose rapidly from9 days and reached to a peak level of 350.6ng/ml at 3 days before ovulation. Whileovulating the level become to 302.2ng/ml. This work provides further evidence for17α 20β P that plays an important role in final maturation of oocyto. 17β-E₂. maypossibly regulate the timing of 17α 20β P surge. Injection of synthetic chum salmon (Oncorhychus keta)gonadotrophin-releasinghormone (s-GnRH) and its analogue (s-GnRH-A), the changes of serum 17β-E₂, Tand 17α 20β P levels are similar to that of natural ovulation. s-GnRH and s-GnRH-Ainjected fish ovulated a week earlier and synchronous than those of control.