Aglepristone (RU 46534) is a competitive progesterone antagonist; it binds to progesterone receptors (PRs) without inducing the molecular cascade associated with progesterone. Its affinity to PRs is higher than progesterone (3.12, 3.8, and 9.26 times greater in the bitch, doe rabbit,and queen, respectively). Aglepristone can therefore beused in various progesterone-dependent physiological orpathologic conditions, with the aim of blocking the action of progesterone. Aglepristone has proven to be an effective and safe means of inducing pregnancy termination or parturition in a large number of domestic species. In domestic species, aglepristone is routinely used for the treatment of pyometra and feline mammary fibroadenomatous hyperplasia, both of which are progesterone dependent. Aglepristone is marketed under the brand name Alizin used as the treatment option for unwanted mating and pregnancy in dogs