The aim of this work was to study the anatomical and pathological reaction and the mechanism of the formation of the pleural symphysis during pleural talcosis. The experiment was performed on fifteen dogs of similar breed, divided into three groups of five subjects each. After thoracoscopy under general anaesthesia, 2 ml of intrapleural physiological saline were injected in group I (controls) and 2 or 4 ml of talc granules in group II and III. A drainage tube was positioned at the end of the examination. One dog in each group was sacrificed on the 1st, 2nd, 7th, 15th, and 30th days post-thoracoscopy. At autopsy a detailed macroscopic study was carried out and some biopsies were taken for histology. In the control group, the inflammatory reaction was very moderate and rapidly disappeared whereas in the groups treated with talc, the talc led to an exudate of several millimeters, the exudate of inflammatory reaction was acute and early (J1) and involved the pleural in particular on the costal surface and was more moderate on the visceral surface and only involved the lung to a thickness of 2 or 3 mm and a few peripheral alveolar spaces. The granulomatous reaction occurred later (from the 3rd day) and was accompanied by the formation of a symphysis by the deposition and coagulation of fibrin which continued from the 7th to the 15th day, and became solid on the 30th day post-thoracoscopy. There was no significant difference between the two groups treated with talc, implying that the reaction was linked to the talc and was independent of the dose used.