Innervation of the human atrioventricular (AV) valves is microscopically studied by histopathological methods. The tricuspid and mitral valves of 4 autopsied hearts of adult men (age range from 50 to 74 years old) without any cardiovascular diseases were stained for acetylcholine-esterase by histochemical method in the medium containing acetylthiocholine iodide. Acetylcholine-esterase positive nerve fibers of 2 to 50 microns in diameter were widely distributed in the subepicardial space of the atrial of the AV valve. They formed a coarse network of the nerve elements from the valve base to the anatomical edge. The nerve network was more dense at the valve ring and base, as well as at the commissure, than at the edge and body. Some thick nerve fibers ran in the chordae tendineae. The thick fibers were intercalated with varicose-like special structures at several places in the leaflets, which seemed to be a kind of sensory apparatus. The thin nerve fibers ended, as usual, at small dot or brush-like apparatus. It is widely accepted that the acetylcholine-esterase positive nerve fibers are identical with vagal nerves which are insisted on participating in development of mitral valve prolapse syndrome. We suggest that the vagal innervation in the AV valves could play an important role for valvular function.