Thermally labile N-hydroxyarylamines, which are the active metabolites of carcinogenic/mutagenic aromatic amines and show potent direct mutagenicity, were studied by fast-atom bombardment (F.A.B.) mass spectrometry and electron-impact (E.I.) mass spectrometry. The protonated molecular ion [M + H]+ and the molecular ion [M]+ were observed at high intensity in the F.A.B. mode. The fragment ions corresponding to [M + H-16]+, [M + H-17]+ and [M-16]+, [M-17]+, [M + H-32]+ and [M-32]+ were also observed characteristically. The quasimolecular ion peaks were shifted up by the numbers of active hydrogens in molecules after the hydrogen-deuterium exchange with [hydroxy--2H3]glycerol and 2H2O. The formation of the ions continued stably throughout the period of measurement, and the decomposition of the samples did not occur in the F.A.B. ion source, compared with the E.I. mode. Hence, it is suggested that the F.A.B. technique is useful for the analysis of the heat-labile toxicologically important N-hydroxyarylamines.