1. St. Louis encephalitis virus injected intracerebrally into susceptible mice multiplies there to reach a titre of 109 intracerebral lethal doses. It is found also in the blood in small amounts immediately following injection and preceding death.
2. Injected intraperitoneally or subcutaneously the virus circulates in the blood for several hours and survives in the spleen for days. It does not multiply in the brain and cause encephalitis, however, unless overwhelming doses are injected or the brain is traumatized.
3. Virus dropped into the nares is demonstrable in the olfactory bulbs at 24 hours, in the piriform lobes at 24 to 48 hours, in the remainder of the brain at 3 days, and in the spinal cord at 4 days. In the brain it reaches a titre of 109 in 6 days. Virus is not readily demonstrable in the blood but is present in the spleen after 48 hours.
4. Virus survives and is capable of multiplying in the spleen.
5. Lesions following nasal instillation of virus appear first in the olfactory bulbs on the 3rd day, in the piriform lobes on the 4th, and in Ammon's horn on the 5th day. The character of the lesions in order of their appearance is exudation of mononuclear cells about superficial blood vessels and in the pia, hyperplasia of the endothelium of the pia, and necrosis of nerve cells of the olfactory tract.