The intracerebral inoculation of hamster neurotropic measles virus produces a fatal acute encephalopathy in the majority of weanling BALB/c mice. The few survivors remain healthy indefinitely. In contrast, weanling SJL/J mice are relatively resistant to acute encephalopathy, and about one-third of the survivors predictably develop a chronic encephalitis many weeks after inoculation. This later illness is characterized clinically by wasting, lethargy, seizures, or focal paralysis, usually culminating in death. Pathologically, a considerable brain inflammatory response occurs in association with abundant measles viral antigen. Viral antigen and inflammation were demonstrable predominantly in the limbic system but was also present in the cerebellum and brain stem. Antibody to measles virus was readily demonstrated in all animals with chronic encephalitis. This report constitutes the first description of chronic measles encephalitis in an inbred host.