Abstract: The diurnal fluctuation of serum gonadotropin concentration during the spawningseason was studied in the common carp and two species of the "domestic carps", the grasscarp (Ctenopharyngodon idellus)and the silver carp (Hypophthalmichthys molitrix). In allthe species similar rhythmic surges are observed, mainly occurring in daytime. However,the peak and valley values of the serum gonadotropin concentration of the commoncarp (20-40 vs. 5-15 ng/ml)are considerably higher than those of the grass and silvercarps (2--8 vs. 0. 1--1 ng/ml). This difference suggests that the secretion of gonadotropinof common carp is high enough for modulating its reproductive activity while, the grassand silver carps cannot spawn, unless exogeneous hormones are given. In the presence of LRH--A, the in vitro GTH relcases of the pituitaries collected inthe night is greater than thet in daytime. The significance of this phenomenon isdiscussed.