Alzheimer's disease (AD) is widely held to be a disorder associated with oxidative stress due, in part, to the membrane action of amyloid~3-peptide (A/3). A~-associated free radicals cause lipid peroxidation, a major product of which is 4-hydroxy-2-trans-nonenal (HNE). We determined whether HNE would alter the conformation of synaptosomal membrane proteins, which might be related to the known neurotoxicity of A~3and HNE. Electron paramagnetic resonance spectroscopy, using a protein-specific spin label, MAL-6 (2,2 ,6,6-tetramethyl-4-maleimidopiperidin-1-oxyl), was used to probe conformational changes in gerbil cortical synaptosomal membrane proteins, and a lipid-specific stearic acid label, 5-nitroxide stearate, was used to probe for HNE-induced alterations in the fluidity of the bilayer domain of these membranes. Synaptosomal membranes, incubated with low concentrations of HNE, exhibited changes in protein conformation and bilayer order and motion (fluidity). The changes in protein conformation were found to be concentration-and time-dependent. Significant protein conformational changes were observed at physiologically relevant concentrations of 1-10 p~MHNE, reminiscent of similar changes in synaptosomal membrane proteins from senile plaque-and A/3-rich AD hippocampal and inferior parietal brain regions. HNE-induced modifications in the physical state of gerbil synaptosomal membrane proteins were prevented completely by using excess glutathione ethyl ester, known to protect neurons from HNEcaused neurotoxicity. Membrane fluidity was found to increase at higher concentrations of HNE (50 /2M). The results obtained are discussed with relevance to the hypothesis of A/3-induced free radical-mediated lipid peroxidation, leading to subsequent HNE-induced alterations in the structure and function of key membrane proteins with consequent neurotoxicity in AD brain.