The nerve root of L7 was exposed, and a clamp was applied to simulate radiculopathy. Intraradicular blood flow was measured at the takeoff point of the nerve root and at the distal to the dorsal root ganglion before and after nerve root infiltration with 2% lidocaine or physiological saline solution (control group), or sympathetic ganglion block with 2% lidocaine. To investigate one of the mechanisms of the therapeutic effect of nerve root infiltration by assessing changes in intraradicular blood flow. Increased intraradicular blood flow was noted both proximal and distal to the clamp after nerve root infiltration or sympathetic ganglion block with 2% lidocaine. No increase was seen after nerve root infiltration with saline solution. Macroscopic and microscopic examination showed that dye after nerve root infiltration did not spread beyond the clamped region to the proximal site. Intraradicular blood flow was measured with a tissue blood flowmeter using the electrolytic hydrogen clearance method before and after nerve root infiltration with 2% lidocaine or physiologic saline solution (control group), or sympathetic ganglion block with 2% lidocaine. Increased intraradicular blood flow was noted both proximal and distal to the clamp after nerve root infiltration or sympathetic ganglion block with 2% lidocaine. No increase was seen after nerve root infiltration with saline solution. An increase in intraradicular blood flow is related to one of the mechanisms of the therapeutic effect of nerve root infiltration. This effect may be mediated by the sympathetic nervous system.