Populations of oligodendroglial cells (ODC) visualized by the neuroglial impregnation technique were investigated in demyelinated (A), actively demyelinating (B) and non-demyelinating (C) zones in both the cerebral and cerebellar white matter of monkeys with active EAE lesions. A marked reduction of the ODC population (P less than 0.001) was recognized not only in zones A and B but also in zone C as compared with those of non-sensitized monkeys. This suggests that the injury seen in ODC precedes myelin sheath destruction.