AbstractThe fine structure of the mitral valve in the chicken heart is described. The chicken has no distinct muscular component within the mitral valve; on the other hand, the tricuspid valve is a muscular fold.The mitral valve is a thin, membranous, fibrous structure and is composed of three layers: (a) an auricular and ventricular endothelial layer, (b) a zona spongiosa, and (c) a zona fibrosa. The endothelium of the heart valve resembles closely that of vascular channels, but the shape and size of the valvular endothelial cells vary considerably from those of other vascular channels. From the valvular endothelial cells thin villus‐like projections extend into the cardiac lumen. Their cellular junctions frequently have a complicated pattern in the form of interdigitating folds, although some show a long linerar zone of simple overlapping between adjoining cells. A fine filamentous plexus is a feature of the cytoplasm but is less evident than in other vascular channels. The basement membrane varies with its location in the valve; it is distinct on the atrial side but is often scanty and obscure on the ventricular surface. Some elastic tissue is found in the subendothelial tissue.The zona spongiosa and zona fibrosa are quite similar to those of other species, but the amorphous ground substance in the zona spongiosa is more abundant. Both layes contain elastic fibers and collagen in both fibrils and bundles. Their amount, composition and configuration vary in different locations within the valve.Avian valvular endothelium is compared with that of lining vascular channels and valves of other species. There are structural differences, some of which may reflect physical conditions and function, such as mobility, extensibility and state of contraction.