Human chorionic gonodotrophin(HCG)has been used in the study of artificial spawning of fishes by many authors 1937;Sneed and Clemens,1959;Tchou and Wang,1962;Anwand,1963;etc). Ovulation and maturation in vivo has also been successfully achieved in the common freshwater cyprinids,Hypophthalmichthys molitrix Cuv.and Val.and Aristichthys nobilis Rich.,by the same hormone during 1958～1963(Tchou et al, 1962)Following a series of works on the culture of female and adult fishes, the induction of spawning of the sexually mature adults and the breeding of young fry,a method for the artificial propagation of these two kinds of fres- hwater fishes was firmly established.This method of artificial propagation was soon popularized throughout our country and the yield of freshwater fishery was greatly promoted.However,the effect of HCG on Ctenopharyngodon idell- us Cur.and Val.,another important freshwater fish of China,was found to be negative by Liu et al(unpublished)and Wu and Chung(1964).The present work was undertaken to reinvestigate the action of HCG on this species of fre- shwater fish. Healthy and sexually mature Ctenopharyngodon with a body weight of ab- out 10.0～15.5 kg.reared in small ponds was used.The chorionic gonadotro- phin was extracted from the urine of pregnant women after the modified Cla- esson's method(Tseng et al,1962)and Got's method(1960).Two HCG preparati- ons with different potency expressed in Bufo unit(Tchou and Wang,1958~ Wang,Tso et al,unpublished)were used for intraperitoneal injection.Artific- ial insemination was performed at about the time of proper maturation and the fertilized eggs were reared in open rectangular containers made of hemp, which were placed in running river on a large scale.The experiments were car- ried out at 22～27℃,and the results obtained can be summarized as follows: 1.Human chorionic gonadotrophin can likewise induce ovulation and ma- turation of oocytes of C.idellus. 2.Within a certain limit,the effect of HCG depends on its purity and the amount injected.Better results can be obtained with purer preparation and in- tensified injection.Thus,the HCG preparation at a concentration of 4.0 mg. in 30cc.Ringer's solution,can induce normal ovulation and maturation of the ovarian fragment of the common toad(Bufo bufo asiaticus),but can not induce ovulation of C.idellus even at a dosage of 80rag.per kilogram.Whereas,anot- her prepartion of HCG at a concentration of 0.5～1.0mg.in 30cc.Ringer's solu- tion can induce ovulation and maturation of the toad oocytes as well as the present fish at a dosage of 10～20mg.per kilogram,obviusly,the failure of so- me authors to affect artificial spawning of C.idellus is probably due either to the non-availability of male and female adult fishes at proper sexual maturity or to the insufficiency in purity and amount of the hormone used,or due to both of these causes. 3.Ovulation takes place 12～15 hours after HCG injection(22～27℃).Incr- ease in the amount of hormone injected will somehow shorten the latent period repuired to initiate ovulation. 4.Cytological study indicates that maturtion division is initiated by human chorionic gonadotrophin.All of the ovulated eggs and sometimes a portion of ov- arian eggs were found to be at the stage of metaphase of the second maturation division upon injecting the hormone(Plate Ⅰ:1～3.Ⅱ:5～8)just like those found in other fishes and vertebrates. 5.The ovulated eggs are mature in the physiological sense,since they can develop normally upon artificial insemination(Plate Ⅰ:4.Ⅱ:9).The percentage of hatchability reached 80% in large-scale experiment,and healthy fries full of vitality were obtained.