2. This hog cholera encephalitis is characterized by a varying amount of vascular and perivascular infiltration with small lymphocytes, mononuclear elements, a few plasma cells, and occasionally a few eosinophilic leucocytes. The glia shows a proliferation surrounding infiltrated vessels or forming small nodules or more diffuse foci. Satellitism and in a few instances true neuronophagia have been observed. Both microglia and macroglia participate in this process. There is no essential increase of glia fibers. In nearly all parts of the central nervous system degenerating lesions of the nerve cells such as tigrolysis and degeneration of the nucleus, including a slight atrophy of endocellular neurofibers, are encountered. No demyelinization has been observed. Specific inclusion bodies in the nerve cells are absent. In addition, in a certain number of cases microscopic and macroscopic hemorrhages are present in the brain, spinal cord, and meninges.

3. These lesions in varying degrees have been found in swine infected with four different strains of hog cholera virus. Two were laboratory strains and two were obtained from fresh field outbreaks.

4. Histological changes in the central nervous system were found as early as 6 days after infection before the animal showed central nervous system symptoms. In two cases which were paralyzed no lesions in the central nervous system could be demonstrated.

5. The lesions in the central nervous system are considered to be the anatomical substratum for the various nervous symptoms commonly found in hog cholera.