Radioimmunoassay (RIA) has been applied for 17β-oestradiol (17β-E₂) in fish serum. Synthetic analogue of hypothalamic luteinizing hormone(LRH-A) has been used as a stimulant for induced spawning in blunt snout bream.Both serum gonadotropin (GTH) and 17β-E₂ levels have been measured by means of RIA. For 17β-E₂ RIA, the accuracy (recovery, 92.8±8.16%)，specificity (crossreactions with oestrone and oestriol < 3.20 and 2.99%)and sensitivity ( Minimum measurement，2.36-2.50pg) have been determined. The standard curve, г=一0.998, s= 0.023-0.051，measurement range was from 10 to 400pg/tube. After injection of LR,H-A, serum GTH level rose to about 17 times of prespawning, while only 3-5 times in unspawning fish. This showed changes in serum GTH were similar to that of grass carp and silver carp during induced spawning. Serum 17β-E₂ levels and gonado-somatic index (GSI) rose synchronally during the development of ovary. At stage IY of ovary, serum 17β-E₂ reached to a peak level of 2004.11±1136pg/ml. That peak level may be supposed as a sign of quick accumulation of yolk in the oocytes. It was found that 17β-E₂ level slightly declined to 13 92. 71±399.09 and 1219.29±420.51pg/ml just before induced injection. Postspacaning, 17β-E₂ sharply decreased to 346.7±129.51 and 324.28±228.00pg/ml. In contrast to male fish, 17β-E₂ levels progressively decreased during the development of testis, and kept in low levels.