The neuronal distribution of gamma-aminobutyric acid (GABA) transaminase (GABA-T), the enzyme which metabolizes GABA, has been mapped in rat brain. The method involves staining for newly synthesized GABA-T by the previously established nitro blue tetrazolium technique in animals killed 8-48 hours after administration of gabaculine, an irreversible inhibitor of GABA-T. Neuronal staining is obscured by staining of other elements if initial suppression is inadequate or survival times postgabaculine are too long. With appropriate conditions, GABA-T-positive neuronal somata can be widely detected. The stained cells include neuronal groups previously reported to be GABAergic on the basis of glutamate decarboxylase (GAD)-colchicine immunocytochemistry and other methods, i.e.: Purkinje, basket, Golgi, and stellate neurons of the cerebellum; basket and stellate neurons of the hippocampus; granule and periglomerular cells of the olfactory bulb; magnocellular neurons of the hypothalamus; and neurons of the striatum, pallidum, entopeduncular nucleus, cortex, medial septal area, diagonal band, substantia innominata, reticular nucleus of the thalamus, substantia nigra, and dorsal raphe. Other cells that stain intensely for GABA-T and may be GABAergic include neurons in the midlateral septal area, accumbens, the central medial and basal nuclei of the amygdala, zona incerta, the brainstem reticular formation, central gray, interstitial nucleus of Cajal, and various thalamic nuclei including the periventricular, intralaminar, rhomboid, and subparafascicular. Known non-GABA neuronal groups are negative for GABA-T staining under these conditions, reinforcing the hypothesis that GABA neurons are far more GABA-T intensive than other neurons.