Internal hydrocephalus can be produced experimentally by injecting a foreign substance into the ventricles. In these experiments, aleuronat, a granular, insoluble material, has caused an acute inflammatory reaction, characterized in the first week by an exudate consisting largely of polynuclear leucocytes. Later the picture is one of a chronic process; polynuclear cells are replaced by lymphoid and large mononuclear cells and there is proliferation of the connective tissue in the choroid plexus. Proliferation of the ependyma occurs in the first week but becomes more advanced in the second and third weeks, and there is increase in neuroglia more marked in the long continued experiments. Ordinarily when aleuronat is injected into a serous cavity, the pleural cavity, for example, abundant accumulation of fluid takes place in twenty-four to forty-eight hours, and at the same time polynuclear leucocytes collect. In the experiments in which the irritant was injected into the ventricle of the brain there was little or no dilatation apparent in the first week; absence of dilatation in all probability is due to the free outflow of the fluid. When obstruction occurs during the chronic stage of the inflammatory process dilatation of the ventricle follows. Choked disc and other symptoms of increased intracranial pressure accompany experimental hydrocephalus. Dilatation occurs slowly and reaches a maximum in about two months. In some of the experiments of longer duration obstruction can be demonstrated in gross or microscopically. Obvious obstruction has not been found by gross examination in all instances, but in the experiments in which India ink was injected into the ventricle before death obstruction to outflow was very readily demonstrated. The third and fourth ventricles were in all instances filled with the pigment, but none appeared on the surface of the brain, whereas in normal dogs the entire surface, especially the base, became deeply pigmented. Obstruction to the circulation of cerebrospinal fluid causing internal hydrocephalus may occur at the foramen of Monro, in the aqueduct of Sylvius, or, doubtless with greatest frequency, at the foramen of Magendie.