Glaucoma is considered to involve neurochemical alterations in the visual system. While the role of excitotoxicity in glaucoma remains controversial, we showed that the balance between glutamate, a main excitatory signal, and gamma-aminobutyric acid (GABA), a main inhibitory signal, is involved in glaucoma pathogenesis. We demonstrated that the visual cortex of glaucoma patients changes to an excitatory-dominant state and that this change is driven by reduced GABA. Additionally, we showed that visual field loss is associated with reduced N-acetyl-aspartate, a marker for neuronal integrity. Taken together, these findings suggest that neurochemical alterations may serve as informative markers for glaucoma.