Adult Wistar-Imamichi rats were induced to superovulate by an i.p. injection of PMSG followed by an i.p. injection of hCG 48 hr later. The doses of PMSG and hCG administered were 0, 75, 100, 150, 300 iu/kg body weight. In this study the doses were based on iu/kg body weight rather than iu/rat to facilitate accurate induction. The following results were obtained. In the adult rats (10 to 15 weeks old) PMSG induced an increase in the number of morphologically normal and fertilized oocytes, as well as in the total number of superovulated oocytes, dose-dependently upto 150 iu/kg. The increase was significant compared with control rats, but PMSG treatment had only half this effect in young rats (6 to 9 weeks old), compared with the adult rats. These results suggest that sensitivity to PMSG changes with age. The proportion of fertilized oocytes among the morphologically normal oocytes was not significantly different in young and adult rats, and both groups had high percentages. In view of the results of this study, the authors recommend that, to collect a large number of fertilized oocytes required for generation of transgenic rats, the induction of superovulation be performed in adult rats aged 10-15 weeks rather than young rats aged 6-9 weeks and giving PMSG at a dose of 150 iu/kg and hCG at a dose of 75 iu/kg.