Rat and cat spinal cords were exposed to 2000, 3000 or 4000 rads of x-irradiation prior to producing an area of primary demyelination in the dorsal columns by the injection of lysolecithin. In animals irradiated with 4000 rads no remyelination by either Schwann cells or oligodendrocytes occurred. With 2000 rads both types of remyelination occurred, but when compared to unirradiated controls, central remyelination was less extensive, while Schwann cells remyelinated a greater percentage of axons. With 3000 rads the results were variable, some animals responded similarly to those in the 4000 rad group, whereas others responded as the 2000 rad animals. Oligodendrocytes were found among the persistently demyelinated axons in the 4000 rad animals and their processes were associated with, but only rarely formed a myelin sheath round, the demyelinated axons. It was concluded that irradiation damage to local cells was responsible for the inhibition of remyelination but it could not be determined if this was due to its effect on the oligodendrocytes alone. The origin of the oligodendrocytes found among the demyelinated axons is discussed in this context.