A diet containing 1.25% elemental tellurium (Te) when fed to rats in week 3 of life produces acute peripheral nervous system (PNS) demyelination. The purpose of this investigation was to determine whether Te has the same neurotoxic effect on Schwann cells and their associated myelin when located within the spinal cord. Schwann cells were induced into the CNS by irradiating the lumbosacral spinal cord of 3 day-old rats with 4000 rads of soft X-rays (Heard & Gilmore, 1980). At 22-28 days of age, a diet containing 1.25% Te was fed to half of these rats, and others were fed on rat chow alone. Non-irradiated rats of the same age were divided into two similarly fed groups. At intervals from 2 to 15 days after the initiation of this diet, the rats were perfused and the irradiated portion of the cord, or comparable level in non-irradiated rats, was trimmed and processed for light and electron microscopy and immunocytochemistry. All rats fed Te became paretic by 6-7 days, and diffuse demyelination with obvious degenerative changes in Schwann cells was seen in the nerve roots. Immunocytochemical localization of Schwann cells and peripheral myelin in the spinal cord was demonstrated using Po antiserum, and in these areas the reduction of astrocytes and their processes was shown using sections incubated with GFAP antiserum. In these areas, as in the roots, there was myelin lamellar separation, and many Schwann cells contained cytoplasmic vacuoles, hypertrophied lysosomal structures and myelin debris. Adjacent oligodendrocytes and CNS myelin were apparently unaffected, confirming a differential susceptibility of oligodendrocytes and Schwann cells. There were no Schwann cell abnormalities in the non Te-fed irradiated rats. This experimental model provides a situation in which other neurotoxic compounds can be evaluated to compare their effects on CNS and PNS myelin in the same milieu.