DBA/2 mice infected with the D variant of encephalomyocarditis virus (EMC-D) (10(1) PFU/head) developed biphasic hind limb paralysis. As a first step in clarifying its pathogenesis, we examined the distribution of viral RNA in the spinal cord using in situ hybridization. At 3 days post inoculation (DPI), in the spinal cord of mice showing slight paralysis, viral RNA was observed in capillary endothelial cells and a few adjacent glia cells in the funiculus lateralis from thoracic to lumbar enlargement. At 7 DPI, in the spinal cord of mice showing apparent paralysis, viral RNA was observed in a larger number of glia cells in the demyelinated lesion associated with infiltration of macrophages in the funiculus lateralis and in a small number of degenerated neurons in the cornu ventrale. In the funiculus lateralis, viral RNA could not be observed after 28 DPI. On the other hand, viral RNA was observed in degenerated neurons in the cornu ventrale of mice showing the second phase paralysis at 42 DPI. Many CD4+T cells infiltrated around these degenerated neurons. These results suggest that: (1) the viral entry zone was the capillary endothelial cells in the funiculus lateralis; (2) first phase paralysis was due to demyelination caused by EMC-D and associated with macrophage infiltration; (3) second phase paralysis was due to degeneration of motor neurons bearing viral RNA associated with infiltration by CD4+T cells.