A method for analysis of aliphatic aldehydes in biological samples is described. Cyclohexanone is added as internal standard and the samples are treated with hydroxylamine and perchloric acid. The oximes are extracted and converted to the oxime-tert-butyldimethylsilyl derivatives, which are quantitated by capillary gas chromatography and identified by mass spectrometry. The characteristic M-57 fragment ions in the mass spectra enabled a rapid identification of the derivatives of the aldehydes, alkanals, alk-2-enals, alka-2,4-dienals, and 4-hydroxyalk-2-enals, which in addition gave rise to characteristic double peaks in the gas chromatographic analysis. The method was applied to analysis of autoxidized arachidonic acid, ADP-Fe3(+)-treated rat hepatocytes, and rat liver given a single dose of ethanol, 5 g/kg. The amounts of hexanal and 4-hydroxynon-2-enal were not increased 6 h after the administration of ethanol.