This study was carried out to investigate the effect of the timing and dose of human chorionic gonadotropin(hCG) on oocyte recovery, in vitro fertilization, and preimplantation development in superovulation of mouse. F1 hybrid($C57BL{ imes}CBA$) mice were obtained and superovulation was induced in female mice by sequential intraperitoneal injection of PMSG and hCG. In the first series of experiments, mice received 5 IU of PMSG given intraperitoneally, and 48 hours later were injected 1 IU, 5 IU, or 10 IU of hCG respectively. In the second series of experiments, mice received 5 IU of PMSG given intraperitoneally and were injected 5IU of hCG 36, 48, or 60 hours later respectively. 1. When the mice received 5 IU of PMSG given intraperitoneally and 48 hours later were injected 1 ItT, 5 IU, or 10 IU of hCG respectively, there were no differences in the total number of the oocytes obtained from the three experimental groups. When the cultures were examined 48 hrs after the termination of insemination the proportion of unfragmented oocytes which had developed over two-cell stage was observer to be lowest in 10 IU hCG group. When the cultrues were examined 120 hour after termination of insemination the proportion of embryos which had developed to the blastocyst stage was observed to be significantly higher in 10IU hCG group than 5IU hCG group(p<0.05), but there was no difference between 10 IU hCG group and 1IU hCG group. 2. When the mice received 5 IU of PMSG and were injected 5 IU of hCG 36, 48, or 60 hours later respectively, there were no differences in the total number of oocytes obtained from the three experimental groups. When cultures were examined 48 hour after the termination of insemination the proportion of unfragmented oocytes which had developed over two-cell stage was observed to be significantly lower in 36 hour interval group than 48 hour interval and 60 hour interval group(p<0.05). When the cultures were examined 120 hour after termination of insemination the proportion of embryos which had developed to the blastocyst stage was found to be higher in 60 hour interval group than 36 interval or 48 hour interval group (P<0.05), and the proportion of hatching blastocyst was found to be higher in 60 hour interval group as well. In this study, it was concluded that the administration of adequate dose of hCG, and long (60 hour) PMSG-hCG interval were necessary in superovulation of mice($C57BL{ imes}CBA$) in order to get a large number of oocytes which had an early oocytes which had an early embryonic developmental capability when fertilized in vitro, and especially it had better have been avoided to administer a large dose of hCG.