Light microscopic autoradiography was used to identify cells in the neostriatum that became labelled after the local injection of [3H]gamma-aminobutyrate (GABA). The GABA-accumulating cells comprised up to 15% of the total population of neurons. Thirty-seven of these cells were examined in the electron microscope and it was found that they all had similar cytological characteristics, i.e., prominent nuclear indentations, a moderate volume of cytoplasm, rich in organelles, and sparse synaptic input to the perikaryon. Nine of the cells that had accumulated GABA were also impregnated following Golgi staining. These Golgi-impregnated neurons were of medium size and all had dendrites that were aspiny, often varicose, and that occasionally followed a recurving path. After gold toning, the Golgi-impregnated, GABA-accumulating neurons were examined in the electron microscope and were found to receive boutons forming symmetrical or asymmetrical synaptic contacts on their somata and dendrites; the symmetrical synapses were most common on the cell body and proximal dendrites, while the distal dendrites mainly received boutons forming asymmetrical contacts. We conclude that one type of GABAergic neuron in the neostriatum is a type of medium-sized aspiny neuron and that this neuron is likely to receive synaptic input both from neurons within the striatum and from neurons in distant brain regions. We suggest that this neuron is a local circuit neuron in the neostriatum since its morphological features are quite distinct from those of identified projecting neurons.