Electron microscopic study of the endothelium of the bacterial valvular endocarditis, induced by the single intravenous inoculation of Streptococcus faecalis in the chicken, revealed cellular changes ranging from the relatively early changes consisting of marked increase of pinocytotic vesicles, much more population of free ribosomes, cystic dilatation, shrinkage and collapse of endoplasmic reticulum and mitochondria, predominant lysosomes, phagocytosis, edematous cytoplasm, pseudopodia multiplication and microvilli deformation of cytoplasmic processes, alienation of the intercellular junctions, and estrangement of basement membrane from the endothelial cells, to the final high alterations consisting of cytoplasn1ic lysis, desquamation, and vegetative formation on the endothelial cells of the mitral valve. These various lesions occurred at 2, 4, 6, 12, and 24 hr, and 2, 5, 7, and 10 days after bacterial inoculation. The changes varied in different locations on the valve. Most of them were found frequently on the auricular surface at the line of closure. The bacterial vegetation developed on the valves of 7 of the 8 chickens from 7 to 10 days after inoculation. The present study has suggested that both bacteria-active and bacteria-passive endothelial cells are present in the valvular endocardium by the selective sensitivity to bacterial injury.