Abstract: The synthetic anaiogue of hypothalamic luteinizng hormone releasing hormone(LRTJ--A) has been used as a stimulant for induced spamning of fishes. Serum gonadotropin(GTH) levels have been measured by means of radioimmunoassay before and behind spa-wning of grass carp (Ctenopharynodon idellus) and silver carp (Hypophthalmichthysmolitrix). The experiments showed that the spawning of the carps would take place onlywhen the serum gonadotropin levels reached to a "threshold valus of ovulation", i. e. about30--40 times higher than that before the inducement. This reveals that the mechanism ofreproductive physiology of fish, is also represented by the hypothalamus-hypophysis-gonad system as well as in other vertebrates.