Animals thought to have hydrophobia should be kept by themselves in a secure enclosure and carefully watched, especially those known to have bitten men or other animals. If a suspected dog have been killed before the suspicion has been confirmed, the head and upper part of the neck should be removed for examination. Veterinarians claim that what they call the plexiform ganglion permits an almost certain diagnosis to be made. The presence of foreign bodies in the stomach of the animal is a corroborative feature. Diagnosis by subdural inoculation requires two or three weeks, and in at least one case a human patient died while waiting for diagnosis to be thus established. _Hydrophobia in man_ is rare in this country, yet is occasionally observed. Its etiology is as yet obscure. That a _contagion vivum_ is present is positive, but its nature is uncertain. Negri, of Pavia, has recently described certain bodies observed in the nervous system of animals dead of hydrophobia which may offer the solution of the problem that has so long been sought. They are found in the protoplasm of nerve cells, but not in their nuclei. They are round or oval in shape, vary in size from 25 microns down to those which can be barely seen with the highest powers. They take ordinary stains. Negri maintains that these bodies are parasites and he has invariably failed to find them in animals which did not have rabies. His work has been confirmed by a number of his colleagues, and bids fair to furnish a reliable and rapid means of diagnosis. The fact that the virus of hydrophobia will pass through a porcelain filter nowise contradicts the view that these bodies may be parasitic, for it is quite possible that they undergo different stages of development, in some of which they are small enough to pass even barriers of porcelain. In fact it seems to have been positively demonstrated that these bodies described by Negri, in 1903, are diagnostic for rabies. They are most likely to be found in the horns of Ammon or the cerebellum. When found here, careful examination must be made of the Gasserian ganglion, where may be found the lesions first described by Van Gehuchten and Nelis, which consist of a proliferation of the endothelial cells to such an extent that the ganglion cells are first invaded and then destroyed, their places being taken by the new cells. The Negri bodies have been generally regarded as protozoa and the specific cause of the disease. At all events, it seems possible always to successfully reproduce the disease in rabbits or guinea-pigs by inoculation with these bodies. If examination shows neither the Negri bodies nor the lesions in the ganglion the presence of the disease can scarcely be suspected, and could only be proved by animal inoculations, which, however, would be advisable in doubtful cases where human beings have been bitten.