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The dynamics of agent replication were studied at 8 levels of spinal cord and in 9 areas of brain of mice infected intraperitoneally with the 139A strain of scrapie agent. Replication in the CNS was first detectable at 2 levels of spinal cord between thoracic vertebrae 4 and 9. The onset of replication was progressively delayed by up to 4 weeks at increasingly lower levels of spinal cord. A similar trend was seen at higher levels of spinal cord and in brain. In brain, agent replication occurred first in medulla, then in the pons and midbrain, thalamus and hypothalamus and, finally, striatum, septum, hippocampus and cerebral cortex. These results are highly suggestive of spread of infection from peripheral sites of agent replication along autonomic fibres to midthoracic cord, followed by an ascending and descending spread of agent at an apparent rate of 0.5 to 1.0 mm/day until the whole CNS is infected. However, experiments involving sympathectomy gave inconclusive results and the evidence for neural spread of scrapie in the peripheral nervous system is circumstantial.