Abstract: A rapid assay for the radioimmunoassay of oestradiol (E2) is develop by using heating to denature protein in serum sample and to replace the chemicals for extraction. This procedure consumes much less time and samples.Levels of E2 in serum of adult female blunt snout bream(Megalobrama amblycephala) were measured throughout the annual reproductive cycle. From January to Apirl, the E, levels increased from a minimun (0.61?.28 ng/ml) to a maximum (5.63?.85 ng/ml). After that period it declined except a slight increase from October to December, measuring around 1.62 ?.20 ng/ml. A positive correlation (r=0.6089) was found between serum E2 level and gonadosomatic index, and the increase of the E2 level was earlier than that of the gonadosomatic index. The peak level of E2 was estimated a month before spawning whereas the average E2 level in spawning season (May to June) was 2.1 ng/ml. The results show that a significant changes of annual serum E2 level in blunt snout bream occurs before spawning and indicate that the E2 is correlated to gonadal development in early spring, but not to its final developmental stage. The slight increase of E2 in autumn may signify the gonadal recrudescence and vitellogenesis. Thus, the present study provides scientific basis for the necessity of the intensive culture in autumn and spring.