The authors describe the ultrastructural alterations of the pulmonary parenchyma produced in dogs by endotoxic shock, and they examine the effects that a ‘secret-olytic’ drug (bromexine) has in modifying those changes. In the animals under shock there is a complete breakdown of the normal structure of the pulmonary parenchyma. According to the authors, these lesions are caused by the damage of the lining layer and of the cells which produce the constituents of the surfactant system. In dogs under shock and treated with bromexine the authors have seen a better organization of the pulmonary parenchyma: the cellular limits of the pneumocytes of types I and II were more clearly defined and the osmiophilic bodies were increased both in number and volume. The authors conclude that the damage of the lining layer and of the pneumocytes of type II plays an important role in the development of the ARDS and they say that bromexine can improve clinical and morphological aspects of that syndrome.