OBJECTIVE: Studies have demonstrated low concentrations of γ-aminobutyric acid (GABA) in the plasma and CSF of individuals with major depression, and low GABA concentrations have also been found in the occipital cortex of depressed subjects. The goal of this study was to determine whether these occipital cortex GABA concentrations are altered after administration of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression. METHOD: By means of proton magnetic resonance spectroscopy, occipital cortex GABA concentrations were measured in 11 medication-free depressed patients before initiation of treatment with SSRI medications and after an average of 2 months of treatment. RESULTS: A significant increase in occipital cortex GABA concentrations was seen after SSRI treatment. CONCLUSIONS: SSRI treatment of major depression is associated with increased occipital cortex GABA concentrations. This appears to result in a normalization of low pretreatment GABA concentrations, an effect that may contribute to a common mechanism of antidepressant action.