Water tests on atrioventricular valves of 125 dead patients with cardiological diseases revealed in 2 of them (1.6%) sagging of mitral cusps in the direction of the left atrium. The valve insufficiency was caused by the rupture of chordal threads in rheumatic desorganization of the connective tissues, necrosis and sclerosis of the papillary muscles in ischemic heart disease. Histological examinations of 104 papillary muscles from 24 autopsy cases showed their frequent involvement, particularly in myocardial infarction of the left ventricle. Under these conditions, comparatively rare development of insufficiency of the atrioventricular valves is explained by their great reserve and compensatory capacities.