Study of viremia in susceptible experimental animals showed all the blood fractions to take part in the infectious process both in acute diseases and in the chronic course of infections caused by various neuroviruses. Thus in animals infected with human acute encephalomyelitis, tick-borne encephalitis, lymphocytic choriomeningitis viruses viremia was observed throughout the entire short period of infection terminating fatally, as a rule. In such cases viremia was of a mixed character, the viruses being found in all blood fractions. The intensity of viremia and the degree of involvement of plasm and blood cell elements into the infectious process may depend upon the animal species, virus strains, form of the disease (acute, chronic). In chronic encephalomyelitis in rats and guinea pigs infected with human acute encephalomyelitis virus viremia are observed for two years (the observation period). In chronic encephalomyelitis blood cells were more actively involved in the infectious process. Virus was found in leukocytes and erythrocytes more frequently than in plasm, though the total amount of virus in the blood was much lower than that in the acute form of this disease. Sequential multiple passages of AEM virus from the blood to the brain in experiments with some strains resulted in intensive viremia thereby demonstrating the possibility of AEM adaptation to the blood tissue.