[Transformation of 19-norgestagens to estrogens by rumen microorganisms]

The 19-norgestagens norethisterone acetate (17 alpha-ethinyl-4-oestren-17 beta-ol-3-on-17-acetate), ethinodiol diacetate (17 alpha-ethinyl-4-oestren-3 beta, 17 beta-diol-3, 17-diacetate), and norgestrol (17 alpha-ethinyl-18-methyl-4-oestren-17 beta-ol-3-on) are transformed to ethinyloestradiol or 18-methyl homologue by microorganisms of cattle rumen. Such transformation of steroid gestagens to oestrogens is likely to offer an explanation for the occurrence of oestrogen effects which had been observed during synchronised oestrus of cattle following oral application of 19-norgestagens.