The effect of Human Menopausal Gonadotropin(hMG) on the implantation, pregnancy, and the concentration of plasma progesterone were studied in pregnant rats. HMG 75 or 150 IU were administered once on day 1, 2, 3, 4, 9, 12 or 14 of gestation, respectively. Rats were autopsied on day 7 or 18. A single dose of hMG prevented implantation and terminated pregnancy in all of the rats by injecting on either day 1 or day 2 and this abortifacient action was effective 82-98% of pregnant rats on day 3 or 4 and 14-20% on day 9 or 12. Administration of hMG had no effect on termination of pregnancy on day 14. Plasma progesterone concentration by injecting hMG on day 1, 2, 3 or 4 were very decreased.