The GABAergic system is the main inhibitory neurotransmitter system in the vertebrate brain. Although it is well established that the GABAergic system is affected in neuropsychiatric disorders, in Alzheimer's disease (AD) it has been considered to be relatively spared. In this study we describe the immunohistochemical localization of the main enzymes of the GABAergic system; glutamate decarboxylase 65 (GAD65), GAD67, and GABA transferase (GABAT) in human brain. In neocortex, hippocampus, basal ganglia, and cerebellum, GAD65 and GAD67 immunoreactivity were found in neuropil granules, possibly axonal boutons or terminals, and in a subset of small to midsized neurons. GAD65 preferentially stained neuropil granules, while GAD67 preferentially stained neuronal cell bodies. GABAT intensely labeled many types of neurons and glia cells. While GAD65 and GAD67 stained the cytoplasm of cells homogeneously, GABAT labeling appeared irregular and granular. GAD65 immunoreactivity of neurons and neuropil was severely reduced in AD middle temporal gyrus, hippocampus, and putamen as determined by fluorescence and light microscopic immunohistochemistry. Western blotting revealed a similar reduction of GAD65, but not GAD67, protein levels in the middle temporal gyrus of AD. Our results suggest that the GABAergic system is more severely affected in AD than previously reported. This deficit may contribute to AD pathogenesis by loss of GABAergic inhibitory activity.