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