Generalized Shwartzman reaction (GSR) was induced in rabbits by two intravenous injections of endotoxin 24 hours apart. Consumption coagulopathy was found in all cases. Histological investigations showed an abacterial endocarditis (thrombotica or verruccosa simplex), comparable to the one seen in humans after shock, in 69% of the treated animals. Biopsies of mitral valve were investigated by electron microscopy. In the early stage of GSR the valvular alterations were characterized by an oedema of the entire valve, an increase in the number of microvesicles in the endothelial cytoplasm, and a marked enlargement of endothelial cell surface by numerous cytoplasmic projections. At a later stage autophagic vacuoles, often showing a destruction of their membranes, and degenerative changes in cytoplasmic organelles occurred in many cells of the valve. On the other hand there were cells in the endothelium, which presented a distinctly increased number of Golgi complexes and of cisternae of rough endoplasmic reticulum. In other regions ruptures of endothelial plasmalemma, as well as hemorrhages into the valvular stroma were observed. Thus predestined loci for thrombogenesis were formed. In conjunction with these structural changes three factors may be involved in the formation of endocarditis due to shock: 1. hypercoagulability in the course of consumption coagulopathy, leading to precipitation of fibrin and platelets in the circulating blood; 2. turbulences of the blood flow, which are produced by the closure of the heart valves, and favour thrombogenesis on valves: 3. direct changes of the endocardium during endotoxemia. Quantitative changes in the different compartments of intracytoplasmic organelles in many cells of the endothelium during the later stage of GSR are discussed.