ABSTRACT After incubation of equilin with rat liver slices, equilenin was isolated and identified by microchemical methods; in addition to equilenin two metabolites were found which were identified as 17β-dihydroequilin and 17β-dihydroequilenin. Under the same experimental conditions, 17β-dihydroequilin was metabolised to equilin as well as to 17β-dihydroequilenin and equilenin. When equilenin was incubated with rat liver slices, only 17β-dihydroequilenin was found as metabolite; no evidence for the formation of equilin was obtained. Incubation of 17β-dihydroequilenin yielded equilenin, whereas 17β-dihydroequilin could not be detected. These experiments show that rat liver contains a steroid dehydrogenase which dehydrogenates equilin to equilenin; under the conditions used this reaction proceeds irreversibly. The interconversion of the ring B unsaturated 17-oxo- und 17-hydroxy compounds is catalysed by the 17β-hydroxysteroid dehydrogenase in rat liver.