Morphological types and populations of glutamate decarboxylase (GAD)-immunoreactive neurons in rat neostriatum (Str) were studied. Str of colchicine-treated animals contained 3 types of neurons immunoreactive for GAD. The first type, which makes up 80-84% of Str neurons, was medium in size and showed moderate intensity GAD-staining. The somatic morphology of the neurons was identical to the medium-spiny projection neuron. The second type, 3-5% of Str neurons, was small to medium in size and was intensely stained for GAD. The somata of the neurons were round or oval and contained a narrow ring of cytoplasm surrounding the nucleus, which often had nuclear invaginations. There were only a few in each section of the third type, which were large, polygonal, and intensely stained, GAD-immunoreactive neurons, including all 3 types, ranged from 85-87% of the total neuron population. The present study indicated that GABAergic neurons in the Str are not a single morphological type and that most Str projection neurons are GABAergic.