Annual variations in serum thyroxine(T 4) and triiodo L thyronine(T 3) concentrations were measured by double antibody radioimmunoassay in the blunt snout bream(Megalobrama amblycephala). Serum T 4 and T 3 levels were elevated and up to peaks during the spring concomitant with the rapid development of gonads, the elevation of serum T 4 was in March early than the T 3, After the reproductive stage, serum levels of T 4 and T 3 declined, serum T 4 showed two small peaks in August and October respectively; Serum T 3 levels were constant throughout the Summer and Autumn (July to November). Serum T 4 and T 3 levels were low during the winter, the minimum levels of both T 4 and T 3 were in December. The physiological significance of thyroid hormone elevations during spring are discussed.