In Swiss mice inoculated subcutaneously with a mouse-adapted strain of scrapie virus, growth of virus occurred first in lymphocytic tissues and submaxillary salivary gland. Then over a period of many weeks virus spread to other tissues, such as lung and intestine. Relatively late in the incubation period of about 24 weeks, virus reached the brain and spinal cord, where the greatest amount of virus was eventually found. In most tissues, high concentrations of virus persisted throughout the protracted course of the infection. Fatal neurologic disease appeared only after such long-continued presence oi large amounts of virus in the central nervous system [CNS]. Astrocytosis was the most conspicuous neurohistologic change in sick mice, especially early in the course of the disease. Scrapie in the mouse is a generalized viral infection of long duration, in which the clinico-pathologic picture is mainly a reflection of slowly manifested effects of the virus in the CNS.