Levamisole was given orally to 3 groups of dogs, totaling 56 animals. Drug administration period varied from 1 to 120 days. All animals were killed and the central nervous system was examined histologically. Lesions were found in 44 of 56 dogs. These consisted of disseminated perivascular cuffing with mononuclear cells throughout the brain and meninges. The severity of the perivascular cuffing varied considerably and could not be correlated to the length of the drug administration period nor to any other variables. There was no damage to the neural tissues associated with the perivascular cuffing. There was evidence of regression of the lesions whether levamisole treatment was continued or not. The changes observed in this study were considered to be related to the known immunomodulating properties of levamisole. The pathogenesis of this change in uncertain.

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