Metandienone (17β-hydroxy-17α-methylandrosta-1,4-dien-3-one) is a 17α-methylated derivative of testosterone. Despite it has been introduced to the market many years ago, it is still one of the most frequently detected anabolic steroids in sports. The metabolism of methandienone in human have already been investigated for a few decades and extensive metabolites have been characterized. However, in some degree, metabolic pathway of metandieone in human body is still unclear. Therefore, our aim is to investigate the reduction of metandienone at the A ring in phase I metabolism.