The detection of aldehydes has become an important measure of lipid oxidation in biological milieu. Aldehyde 2,4-dinitrophenylhydrazones are easily prepared and readily purified by HPLC and/or TLC and have proven useful for the detection of aldehydes. The lower limit of detection for dinitrophenylhydrazones was significantly reduced by using gas chromatography-mass spectrometric (GC-MS) techniques. Individual dinitrophenylhydrazones were readily separated by GC and detected by both positive and negative ion MS. The two major ions in negative ion spectra were the 182 m/z fragment ion and the molecular ion. Positive ion spectra showed strong ions corresponding to the protonated molecular ion and a protonated iminium ion. The greatest sensitivity was obtained with negative ion detection (10 pg per injection). However, more structural information was obtained from analysis of the positive ion spectra. Dinitrophenylhydrazones of hydroxyaldehydes, like 4-hydroxynonenal, were analyzed after converting the dinitrophenylhydrazones into trimethylsiloxylethers. GC-MS with negative ion detection was used to identify and quantitate the release of 4-hydroxynonenal by alveolar macrophages exposed to nitrogen dioxide.