Thiamine- and vitamin B-deficient encephalopathy was produced in adult Wistar rats and the effect of magnesium deprivation and/or the administration of guanidine, a magnesium antagonist, was studied. These encephalopathic rats had symmetrical lesions, edema, exudation of fibrin, spongy changes, petechial hemorrhages, neuronal degeneration and gliosis in the pontine tegmentum, mainly the lateral and medial vestibular nuclear areas. Mild pathological changes were seen in the purely thiamine- or vitamin B-deficient rats, whereas severe pathological changes occurred in thiamine- or vitamin B-deficient rats combined with magnesium deprivation and/or guanidine administration. However, magnesium-deficient diets or guanidine administration did not produce pathologic changes in rats. These findings suggest that thiamine deficiency is an essential factor in the development of these pathological changes in the brainstem, and that magnesium and guanidine, an antagonist of magnesium, play important roles in the development of these disorders.