In order to provide a basis for studying the molecular mechanism of pharmacological action of local anesthetics and to develop a fluorescence spectroscopic method which can detect the microviscosity of native and model membranes using intramolecular excimerization of 1,3-di(l-pyrenyl)propane (Py-3-Py), we examined the effect of lidocaine·HCl on the microviscosity of model membranes of phosphatidylcholine fraction extracted from synaptosomal plasma membrane vesicles (SPMVPC). The excimer to monomer fluorescence intensity ratio (I'/I) of Py-3-Py in liquid paraffin was a simple linear function of T/η. Based on this calibration curve, the microviscosity values of the direct probe environment in SPMVPC model membranes ranged from 234.97⁑48.85 cP at 4oC to 19.21⁑1.11 cP at 45oC. At 37oC, a value of 27.25⁑0.44 cP was obtained. The lidocaine·HCl decreased the microviscosity of SPMVPC model membranes in a concentration-dependent manner, with a significant decrease in microviscosity value by injecting the local anesthetic even at the concentration of 0.5 mM. These results indicate that the direct environment of Py-3-Py in the SPMVPC model membranes is significantly fluidized by the lidocaine·HCl. Also, the present study explicitly shows that an interaction between local anesthetics and membrane lipids is of importance in the molecular mechanism of pharmacological action of lidocaine·HCl.