The interruption of early pregnancy in the rabbit by oral administration of ethinyl oestradiol (ee) has been reported by Parkes, Dodds & Nobles (1938). Recent work by Chang & Harper (1966) has shown that rabbits fed with various oestrogens, especially ee, 1 to 3 days after insemination will induce the degeneration of blastocysts and that the effect is due to the rapid transport of eggs from the tube to the uterus and their expulsion from the uterus. Chang (1966) has further reported that administration of an orally effective progestin, medroxyprogesterone acetate (MPA, Upjohn Company), for 3 days before ovulation has an effect similar to that observed following the administration of oestrogen after ovulation in the rabbit. This communication describes an experiment concerning the fertilization and transport of ferret eggs following oral administration of MPA before ovulation, or of
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