ABSTRACT
Anti-implantation agents ara characterized as drugs or methods that have their contraceptive effects between fertilization and the end of the impllantation period (days 13-14 post-fertilization). Such agents con be used im post-coital emergency, for repeated post-ovulatory use and/or for menstrual regulation process. Menstrual regulation can be achived by antiorigestins that block progesterone receptors and interfere with the preparation of the endometrium for implantation, by luteolysis causing decreased progesterone levels and interruption of the implantation process, by the uterotonic effects of prostaglandinas. RU 486 administered during yhe luteal phase induces vaginal bleeding ind both women and non-human primates and prevention of prgenancy might be possible by their use as a once-a-month menstrual regulator. As yet, no post-coital agent has been satisfactorily identified and other Anti-implantation agents are being considered