In the search for new metyrapone derivatives as radioligands for the functional diagnosis of adrenal pathology, 2-methoxyphenylmetyrapone [2-MPMP,1-(2-methoxyphenyl)-2-methyl-2-(3-pyridyl)-1-propanone] ( 1), and related 2-substituted phenylmetyrapone derivatives, have been separated as potent inhibitors of adrenal 11β-hydroxylase, with high affinity for adrenal mitochondrial binding sites. Surprisingly, 2-[ 11C]MPMP showed a rapid loss of the radioactive label, which prompted investigation of its metabolism. Synthetic 2-MPMP ( 1) and its seven potential metabolites ( 2–8) have been identified spectroscopically ( 1H- and 13C-NMR and mass spectrometry) and further characterised by chromatography (TLC and gradient reversed-phase HPLC). Chromatographic and mass analysis of urinary extracts from rats dosed with 2-MPMP have confirmed the major metabolites as 2-hydroxyphenylmetyrapone (2-OHPMP), 2) and its N-oxide (2-OHPMP-NO. 6), which are present predominantly as conjugates.