Ovulation in Immature Rats in Relation to the Time and Dose of Injected Human Chorionic Gonadotropin or Pregnant Mare Serum Gonadotropin

The ovulatory response of immature rats to varying doses of human chorionic gonadotropin (hCG), 1–100 IU or pregnant mare serum gonadotropin (PMSG), 3–60 IU, was studied at 24, 48 and 72 h after injection of the gonadotropin. Both hCG and PMSG induced ovulation by 24 h in the absence of significant increases in ovarian weight; the highest percentage ovulation at 24 h was seen with 50 IU hCG. At 48 h, 100 IU hCG or 60 IU PMSG produced a higher percentage of ovulations than the same doses at 24 h. This was in contrast to the unchanged or decreased response seen with lower doses of either gonadotropin. The doses of hCG or PMSG which produced relatively few ovulations at 48 h yielded high percentages of ovulations at 72 h, while doses of the gonadotropin giving a relatively high percentage ovulations at 48 h produced lower percentages at 72 h. These data indicate that the ovulatory response of immature rats to exogenous gonadotropins may involve more than a single mechanism.