The effects of lidocaine, an amide local anesthetic, on the electrophysiologic properties of the nerve cell soma were studied on isolated superfused superior cervical ganglia of rats. Administration of 400 nmol of lidocaine to the preparation evoked significant reversible changes in the membrane properties of the nerve cell soma. The firing threshold, the action potential duration and the intracellular current threshold for firing the cells increased significantly (p less than 0.01), while the action potential amplitude decreased significantly (p less than 0.01). The resting membrane potential change was not significant. The membrane resistance decreased by 20% (p less than 0.01). The change in membrane resistance, and the spike potential evoked by excitatory postsynaptic potential or intracellular current injection were the last to recover after application of lidocaine. The results show that the postsynaptic cell body is a major site of action of the local anesthetic lidocaine, and that it is more sensitive to the action of lidocaine than the presynaptic nerve fibers. The implication of the results is that when local anesthetics are injected to areas where cell bodies and processes (axons and dendrites) are present together, such as during stellate ganglion block, lumbar sympathetic block, celiac plexus block and intrathecal administration for spinal anesthesia, the cell bodies and the processes are all affected, and their excitability and function suppressed. Further, the duration of the block will be determined by the duration of lidocaine-induced cell block rather than the duration of lidocaine-induced nerve fiber block.