The distribution of radioactivity in the blood, the liver, the central and peripheral nervous systems and in the proximal muscles of the hindlimbs was studied after injection of tritiated tetanus toxin into the triceps muscle in guinea pigs. Different doses of toxin ranging from 200 to 200,000 DLM were used and the animals were killed at various times during the development of tetanus disease. It is concluded that the toxin may use a combination of nervous and hematogenous routes for its transport to the central nervous system (CNS). The occurrence of toxin in various parts of the CNS coincides well with the development of the signs of disease. A barrier to the passage of tetanus toxin from the spinal roots into the spinal cord seems to exist.