The effect of GnRH alone and concomitant with PMSG on the prevention of implantation. termination of pregnancy, and concentration of plasma progesterone were studied in pregnant rats. GnRH 50, 100 or 200 ug alone and concomitant with PMSG 25 or 50 IU were administered once on day 2 or 9 of gestation, respectively. Rats were autopsied on days 7 or 20. Administration of GnRH on day 2 did not result in the prevention of implantation and termination of pregnancy but resulted in termination of pregnancy administering on day 9. Administration of GnRM concomitant with PMSG on day 2 or 9 resulted in prevention of implantation and termination of pregnancy, but injection of GnRH 50 ug concomitant with PMSG 25 IU on day 9 had only one live fetus. Administration of GnRH alone and concomitant with PMSG on day 2 had no effect on the concentration of plasma progesterone determining on day 7. Administration of GnRH concomitant with PMSG on day 2 resulted in decrease of progesterone level determining on day 20 but GnRH alone was normal level. Administration of GnRH alone and concomitant with PMSG on day 9 resulted in decrease of the concentration of progesterone but was normal concentration administering GnRH 50 ug concomitant with PMSG 25 IU.