The A. reviewed the bibliography of pathology of murine leprosy. Out of 41 rats expontaneously infected with murine leprosy 39% had alopecia, especially on the back; 78% had subcutaneous infiltrations; 13 of them presented tumour-like grows, sometimes large as 5 cm. of diameter; eight animals had ulcerations varying from 1 to 15 in number; 11 had skin nodules; two presented splenomegaly and two others small nodules upon the surface of the spleen; five had microabcesses on the liver; two had pneumonia and two others showed microcabcesses on their lungs. The other organs were macroscopically normals. Histopathological studies were made with materials of natural and experimental rat leprosy disease. Granulomata were seen on sections of their skins, lumph nodes, spleens, bone marrows, livers, lungs and kidneys. The testicles were rarely envolved. The granulomatous tissues are formed by mononuclear cells or by large plae cells resembling epithelioid celss, in which there were numerous bacilli. The infections process become localisated by long time in the lumph nodes. Rat leprosy by its nature and probably origin of the affected cells suggest a primordially disease of the reticulo-endothelial system.