Abstract: The gas chromatography/mass spectrometric properties of trimethylsilyl ether andtrifluoroacetyl ester derivatives of trenbolone,combined with a methoxine group ,are presented.Thecleavage mechanism and some derivatization problems were observed and discussed.The positive urineof trenbolone acetate was checked with the results of epitrenbolone and possible hydroxy-trenbolone asmain metabolites.Also a little trenbolone and possible estra-4,9,11-trien-3,17-dione were found.