The metabolism of stanozolol (17 beta-hydroxy-17 alpha-methyl-5 alpha-androstano[3,2-c]pyrazole), an androgenic-anabolic steroid widely used in sport for the purpose of enhancing performance, was investigated in humans. The analysis method was based on the use of solid-phase extraction on the Sep-Pak C18 cartridge, enzymic hydrolysis of steroid conjugates and high-resolution gas chromatograph-mass spectrometric (GC-MS) analysis of trimethylsilylated steroid extracts. After administration of a single 20-mg oral dose, twelve metabolites including unchanged stanozolol were recovered predominantly from the conjugated steroid fraction and characterized by GC-MS. In the unconjugated fraction, 16 alpha-hydroxystanozolol, 17-epistanozolol, stanozolol and 3'-hydroxy-17-epistanozolol were the most abundant metabolites. In the aglycone fraction, 16 alpha- and 16 beta-hydroxystanozolol, stanozolol and 3'-hydroxystanozolol were the most abundant metabolites. Other metabolites resulted from regioselective hydroxylation of stanozolol at C-4, whereas other were 17-epimers of 3'- and 16 alpha-hydroxystanozolol. Further hydroxylation leading to the formation of four isomeric dihydroxylated metabolites was also observed. They were tentatively assigned the structures of 3',16 alpha-, 4 beta,16 alpha-, 3',16 beta- and 4 beta,16 beta-dihydroxystanozolol. The mass spectral features of their bis-N,O-trimethylsilyl derivatives obtained under electron-impact ionization are presented. The effect of pH on the relative recovery of some of these metabolites is also presented. The usefulness of this analytical methodology for the detection and identification of stanozolol urinary metabolites in doping-control situations is demonstrated. The metabolism of stanozolol is also discussed, and metabolic pathways accounting for the formation of its biotransformation products are proposed.