The pathological changes that follow a severe infection with Bartonella muris in the adult or young albino rat are: First, those that result from the release of large quantities of cellular debris in the circulating blood. There is phagocytic activity and hyperplasia of the endothelial elements of the liver, thymus, lymph nodes and, in the young rat, of these elements in the spleen, with resultant capillary thromboses and focal necroses. Second, changes result from the anemia as such-fatty metamorphosis of heart, liver and kidneys. Third, there is a severe nephrosis and in some instances a degenerative process in the glomeruli. In the bone marrow hyperplasia of erythropoietic elements occurs.