Superovulation was induced in mature nulliparous Japanese voles (Microtus montebelli) with PMSG as a follicle stimulating factor and hCG as an ovulation inducing agent. PMSG was injected at priming dose levels of 25, 50, 75 and 100 IU respectively, and the dose of hCG used as the ovulator was injected additionally at a high or low level for each dose of PMSG. The results obtained are as follows:
1. When voles were treated with an additional injection of 5 or 10 IU of hCG after pretreatment with 25 IU of PMSG, the average number of ovulated ova was 5.4 and 3.4 respectively, almost the same as the 2.3 of the intact group.
2. The average number of ovulated ova was 6.8, when 50 IU of PMSG and 10 IU of hCG were administered, but when 50 IU of PMSG and 20 IU of hCG were injected the average number of ovulated ova increased to 12.5. There was no significant difference in the average number of ova between the groups given 50 IU of PMSG and the other groups.
3. When 75 IU of PMSG and 15 IU of hCG were administered, the count was 12.4. The largest average number of ovulated ova in this experiment was 14.0, when 75 IU of PMSG and 30 IU of hCG were injected. The average number of ovulated ova was significantly larger in the group with the additional injection of 30 IU of hCG pretreated with 75 IU of PMSG than in the group receiving the injection of 25 IU of PMSG or in the intact group (P<0.05).
4. When the dose of PMSG was increased to 100 IU, an average of 11.1 ova were obtained after additional treatment with 20 IU of hCG and 11.0 ova after additional treatment with 40 IU of hCG, indicating a decrease in the response to more than 75 IU PMSG.
5. Further experiments are necessary to increase the number of ovulations and to decrease the variation in the response to gonadotropin treatment.