Abstract: A radioimxnunoassay (RIA) for annual cycle of gvnadotropin (GTH) pituitary content in common carp has been made. Carp pituitary glands dried by acetone were extracted with 40% ethanol and lyophilized for assay. The annual mean content of gvnadotropin in pituitary gland is 56 .1 μg/mg. GTH pituitary content of both male and female reach to maximum value in February near the ovulation stage (female 154.0 μg/mg，male 160.0 μg/mg)，While GTH pituitary content of male fish in September and that of female in October drop to minimum value 8，3 μg/mg for male and 3.8 μg/mg far female. Later on the contents of both male and female begin to increase gradually until a maximum value is reached again in next February.Seasonal variation of the GTH contents ehibit a definite cycle change and it is positively cari.related with gonadosomatie index (GSI).