The extremely broad spectrum of the biological effects of aldehydic lipid peroxidation products has necessitated the development of a technique that can quantitate all of the aldehydes formed in biological materials. The proposed method is based on the use of O-(2, 3, 4, 5, 6-pentafluorobenzyl) hydroxylamine hydrochloride (PFBHA.HCl) to form the O-pentafluorobenzyl-oxime (PFB-oxime) derivatives of 22 saturated and unsaturated aldehydes (C2-C12) including hexanal, 4-hydroxy-non-2-enal (HNE), and malondialdehyde (MDA), followed by trimethylsilylation of the hydroxyl group to trimethylsilyl (TMS) ethers. The PFB-oxime-TMS derivatives were analyzed by capillary column gas chromatography-negative-ion chemical ionization mass spectrometry (GC-NICIMS) with ammonia as reagent gas. Quantitation was achieved using benzaldehyde-ring-D5 as an internal standard in selected ion recording (SIR) mode. Standard curves were linear (r > 0.99) for all individual aldehydes. The detection limit was between 50 and 100 fmol per 1 microliter injected aldehyde. Recovery of all aldehydes from urine, plasma, and tissue homogenate was over 85%, except HNE, trans-2-octenal and trans-2,-cis-6-nonadienal from plasma and tissue sample, which were between 60 and 80%, suggesting these aldehydes may bind to protein and lipid components, especially to SH groups of proteins. The high sensitivity of this method allows the measurement of physiological aldehyde levels in biological samples. The products of aldehyde metabolism can also be measured by this assay.