To investigate the roles imposed on astrocytes for glutamate metabolism, a specific inhibitor of glutamine synthetase (GS), methionine sulfoximine (MSO), was repeatedly administered to rats and histopathological changes were correlated with glycogen accumulation and the immunocytochemistry of GS and glial fibrillary acidic protein (GFAP). Prolonged MSO-loading (every 12 h up to three times, 100-150 mg/kg body weight) brought about the appearance of astrocytes with swollen, watery nuclei reminiscent of Alzheimer II glia chiefly in the neocortex, hippocampus and lateral thalamus after 24 h. Concomitantly, profound accumulation of glycogen ensued in the superficial three layers of the neocortex, hippocampus and pyriform cortex. GS immunoreactivity appeared enhanced in the cortex, hippocampus and lateral thalamus with parallel increase in GFAP immunoreactivity after prolonged treatment. Oligodendrocytes in the diencephalon and brain stem also normally contained GS immunoreactivity. Some animals developed necrotic lesions in the dorsolateral neocortex. The area of glycogen accumulation coincided with the known distribution of N-methyl D-aspartate (NMDA) glutamate receptors and, thus, GS may play important roles in NMDA receptor-mediated glutamate metabolism. The Alzheimer II type changes, however, did not correlate with NMDA-receptor distribution. These results indicate certain regionalizations in the roles of astrocytes and oligodendrocytes in glutamate and ammonia metabolisms.