This study examined the mechanism of action of a local anesthetic, lidocaineㆍHCl. Energy transfer between the surface fluorescent probe, 1-anilinonaphthalene-8-sulfonic acid, and the hydrophobic fluorescent probe, 1,3-di(1-pyrenyl) propane, was used to determine the effect of lidocaineㆍHCl on the thickness (D) of the synaptosomal plasma membrane vesicles (SPMV) isolated from the bovine cerebral cortex, and liposomes of the total lipids (SPMVTL) and phospholipids (SPMVPL) extracted from the SPMV. The thickness (D) of the intact SPMV, SPMVTL and SPMVPL were 1.044±0.008, 0.914±0.005 and 0.890±0.003 (arbitrary units, n=5) at 37oC (pH 7.4), respectively. LidocaineㆍHCl decreased the thickness of the neuronal and model membrane lipid bilayers in a dose-dependent manner with a significant decrease in the thickness, even at 0.1 mM. The decreasing effect of lidocaineㆍ HCl on the membrane thickness might be responsible for some, but not all of its anesthetic action.