This paper describes electron microscopic observations on early changes in the endocardium, especially of the endothelial cells, and the basement membrane by means of the activity of virus particles found incidentally in the cardiac valve of apparently normal chickens injected with Streptococcus faecalis, during a study on experimental bacterial endocarditis. Aggregation of virus particles was found in the focal edema around the basement membrane of the subendothelial region. Various alterations in the basement membrane were found from the subendothelial focal edema, and Tanged from fibrillation, lamellation, fragmentation, segmentation, and disruption to disappearance. Most of them found in the auricular surface of the mitral valve were more intense than in the ventricular surface. Only the subendothelial fibrillogenesis in relation to the basement membrane was more marked in the ventricular surface. The degenerative changes of the endothelial cells such as shrinkage, collapse, apparent necrosis, and desquamation were seen more frequently in the auricular surface. These endothelial cellular changes appeared to depend on the reaction of the basement membrane for the subendothelial focal ellema considered to be the initial lesion in the vegetative endocarditis.