The effect of GnRH and/or hCG on the implantation, pregnancy, and the concentration of plasma estradiol and progesterone were studied in pregnant rats. GnRH 50, or 100ug and/or hCG 50 or 100 IU were administered once on day 2 or 9 of gestation, respectively. Rats were autopsied on days 8 or 16. Administration of GnRH on day 2 did not induce the prevention of implantation and termination of pregnancy but was able to induce termination of pregnancy administering on day 9. Administration of hCG induced delayed implantation on day 2 and termination of pregnancy on day 2 and 9. Administration of GnRH concomitant with hCG had no effect on prevention of implantation on day 2 but induced termination of pregnancy with a very increased fetal resorption on day 2 and with a moderate increased fetal resorption on day 9. Administration of GnRH concomitant with hCG on day 2 induced more increased termination of pregnancy compared to injection of GnRH or HCG and opposite result was observed on day 9. Plasma estradiol and progesterone concentrations by administering GnRH and/or hCG had no effect on the termination of pregnancy the pregnant rats.