The determination of methandienone (I) (17 alpha-methyl-17 beta-hydroxyandrosta-1,4-dien-3-one) in human urine by gas chromatography and capillary column gas chromatography--mass spectrometry has been studied. After oral administration to man two major metabolites were detected, the structures of which have been identified as 17-epi-methandienone (II) and 6 beta-hydroxy-17-epi-methandienone (III). These metabolites are exclusively excreted in the unconjugated form. At least two more metabolites are extractable from the free fraction of the urine but no measurable amounts of I itself were found. The rate of metabolism and urinary excretion seems to be reasonably fast. The total amount of recovered I in the form of the metabolites II and III is about 5%. Extraction and clean-up procedures and chromatographic details are presented.