From previous experiments and this report, it appears that the capillaries and small blood vessels are the most sensitive organs in shock situations. The early histologic changes detected by electron microscopy and reported here confirm this observation. This finding also helps to distinguish experimentally between the two phases of shock. The vesiculation, intracapillary fibrin formation, disseminated thrombi, and associated metabolic disturbances are considered to be the initiators of irreversible shock. The cardiovascular management, therefore, occupies the utmost important place in this complex syndrome.