The presence of nerve fibers was investigated in porcine and human atrio-ventricular valves by AChE technique, formaldehyde-induced fluorescence, en bloc silver and gold chloride impregnation and electron microscopy. Elaborate nerve plexuses were observed in every leaflet and in some chordae tendineae of all the samples examined, without significant species differences in the pattern of innervation. The presence of a sensory innervation was inferred from the demonstration, in whole mount samples processed for acetylcholinesterase, of thick myelinated nerve fibers and of endings with dot-like or brush-like appearance. Moreover the results of the combined histochemical and ultrastructural methods showed the existence of both cholinergic and adrenergic efferent nerve fibers. Nerve varicosities with clear or dense-cored vesicles were frequently observed in proximity to blood vessels and to cardiac and smooth muscle bundles, which therefore can be considered as the targets of the efferent nerve supply. The complex pattern of the innervation herein demonstrated suggests the existence of a nervous control of valvular function through the regulation of contractile elements.