Summary 1. Many and varied attempts to cause mouse-brain-adapted yellow fever virus of the French strain, after the 146th passage, to again acquire the general viscerotropic properties of the natural yellow fever virus in Macacus rhesus have met with failure regardless of the site of inoculation employed, the time of the subinoculation of the virus, the kind of tissue used as a virus-containing inoculum, and the amount of virus injected. Subcutaneous or intraperitoneal inoculation of the mouse-brain-adapted virus in susceptible monkeys has not led to a reëstablishment of the general viscerotropic properties of the natural virus whether it was transferred from monkey to monkey at 24-, 48-, 72-, 96-, and 120-hour intervals, at the time of fever, or at the death of the animal. 2. Intracerebral or intraspinal inoculation of Berkfeld N filtrates containing the mouse-brain-adapted yellow fever virus in M. rhesus was followed by an encephalitis of characteristic clinical symptoms, with definite pathological lesions, and of a causal relationship to yellow fever virus demonstrated by specific immunological reactions. 3. The occurrence of yellow fever virus encephalitis is accompanied by a widespread distribution of the virus in the peripheral nervous system. At the time of death the virus can be recovered from the brain, spinal cord, retina, salivary glands, sciatic nerve, radial nerve, ulnear nerve, and adrenal glands. In our experience it has not been possible to isolate the virus at death from the blood, spinal fluid, liver, kidney, pancreas, or spleen. 4. The yellow fever virus encephalitis of Macacus rhesus is characterized by the widespread infiltration of mononuclear leucocytes and lymphocytes both in perivascular concentrations and in diffuse disseminations throughout the central nervous system. The infiltration appears to be most marked in the regions of the basal ganglia and brain stem but this is not necessarily the rule. Degenerative changes occur in the neurone cells of the cerebral cortical gyri, brain stem, and spinal cord. 5. The mouse-brain-adapted yellow fever virus of the French strain, after various periods of life up to 7 months in the nervous tissues of Macacus rhesus retains the essential characteristics of mouse-brain-adapted virus to such an extent that the results of its inoculation intracerebrally in mice are indistinguishable from those following inoculation of the pure-strain, mouse-brain-adapted virus of similar passage in mice alone. 6. The mouse-brain-adapted yellow fever virus may be maintained in monkeys by direct transfer through an intracerebral or an intraspinal route of a Berkfeld N filtrate of a suspension of triturated infective monkey brain in 10 per cent normal serum. 7. The neurotropic virus of monkey source may be preserved for at least a period of months by desiccation in vacuo in the frozen state of Berkfeld N filtrates of infective monkey brain suspensions.