Studies have been carried out to standardize induced luteolysis model systems ƵƟůŝnjŝnŐ female monkeys and pregnant rats. In monkeys, ĂĚmŝnŝƐƚrĂƟŽn of a single ŝnũĞcƟŽn of GnRH receptor antagonist, Cetrorelix (CET; 150 µg/kg BW s.c.,), on day 7 of the luteal phase led to profound decrease in serum progesterone (P4) cŽncĞnƚrĂƟŽn within 24 h (3.6 ± 1.1 vs 0.8 ± 0.2 ng/ml before and 24 h post CET, rĞƐƉĞcƟvĞůy p<0.05), and followed by the premature onset of menses 96 h later.