To reveal functional heterogeneity among GABAergic interneurones in rat neostriatum, we investigated: (1) whether nitric oxide synthase (NOS)- and calretinin-immunoreactive cells, like parvalbumin-immunoreactive cells, were also immunoreactive for 67 kD glutamic acid decarboxylase (GAD67) and GABA; (2) whether NOS cells, calretinin cells and parvalbumin cells belonged to separate populations or not. NOS cells, calretinin cells and parvalbumin cells all showed immunoreactivity for GAD67 in colchicine-treated rats. Calretinin cells and parvalbumin cells were also immunoreactive for GABA. Only a few cells (0-3%) showed immunoreactivity for two antigens. These results suggest that the neostriatum has at least three GABAergic interneuronal systems.