In the atrioventricular valves of the hearts of 161 rabbits the authors found permanent spontaneous automatism activity. Isolated leaflets of these valves were examined with the aim to detect the electrophysiological and morphological characteristics of pacemaker formations constituting the substrate of spontaneous automatism. It was found that these formations consist of pacemaker cells of three types: P-cells, cells of intermediary type and Purkinje-type cells. Further, it was found that a decisive role in the genesis of spontaneous excitations of pacemaker cells of AV valves is played by the slow Na-Ca channel. Examination documented good innervation of the pacemaker formations, and the specific effect of mediators [acetylcholine, adrenaline, noradrenaline] on their automatism was investigated. Investigations on isolated rabbit hearts evidenced that impulses originating from the heterotopic excitation focus, localized in the AV valve leaflets, can be conducted into other compartments of the heart and can cause cardiac arrhythmias.