The topographical distribution of gamma-aminobutyric acid (GABA) in the substantia nigra of a 28-year-old male 4 h after death was investigated. In a preliminary study the entire substantia nigra was dissected from transverse sections. The results showed that there was no correlation between the GABA concentration and the number of melanin-rich nigral cell bodies. This was especially so in the rostral third of the substantia nigra. Using the method of Miyata and Otsuka, transverse sections (150 mum) of the rostral, middle and caudal substantia nigra were cut into 500 mum X 500 mum square blocks which were assayed for GABA by an enzymatic method. In the rostral substantia nigra the GABA distribution was markedly uneven. The highest concentration of GABA was found in the pars reticulata. Within the pars reticulata the highest levels of GABA clearly occurred in two separate regions, a medial and a lateral. In the middle and caudal substantia nigra the GABA distribution was again uneven; however, the highest GABA levels were divided between the pars reticulata and the pars compacta. The results support the view that in the substantia nigra the greatest part of the GABA content is due to the presence of striato-nigral nerve terminals which are known to synapse with the dendrites of the substantia nigra dopamine neurons. In the rostral substantia nigra the concentration of GABA within the pars reticulata is in keeping with the presence of dendrites of such neurons in this region. Presumably on this basis it can be assumed that in the middle and caudal substantia nigra the dendrites are oriented in a more rostro-caudal direction.