This is a clinical and pathological report on an unusual case of periarteritis nodosa in which the disease was confined chiefly to the brain and spinal cord. The diffuseness, variability and intermittency of the neurological signs and symptoms over a period of two years, in the absence of many of the more usual somatic manifestations of periarteritis nodosa, was responsible for the nature of the disorder not being diagnosed before death. Mental symptoms were prominent throughout the last year of illness. T.A.B. pyrexial therapy coincided with a marked remission in physical and mental symptoms, whilst arsenical therapy appeared to precipitate the terminal exacerbation of symptoms.