Dose-response inhibition curves for serial dilutions of sera and pituitary homogenates f rom the bag rid catf ish M .macropterus (Bleeker)parallel that for the common carp grow th ho rmone (cGH), so the radioimmunoassay for cGH w as used to assay the blood and pituitary GH levels of the catfish .Seasonal changes w ere found in both pituitary GH content s and serum GH levels .Seasonal changes in pituitary GH contents w ere marked by tw o distinct peaks , one w as in March and the other was in August , just anterio r to and posterior to the gonadot ropin (GTH)peak(April to July)of the pi tuitary .From April to July , the reproductive period of the fish , and f rom November to February , the overw inter period , the pituitary GH content s w ere low and w ith mino r variat ions .Influenced by w ater temperature and pho toperiod , the lowest level of serum GH w as observed in w inter and a prog ressive increase w as found in spring and summer . But a sig nificant increase w as found at the end of summer (July), and remained a high level in autumn .Seaso nal changes in serum GH levels were also co rrelated to reproductive cycle .The lowest serum GH level w as observed in the resting period , elevated g radually with the recrudescence of the gonads , increased sig nificant ly at the spaw ning period , and got the highest v alue in the sexual reg ressed period .These results were in accordance w ith those obtained in common carp and g oldf ish .