GABAergic neurons were localized in the rat basal ganglia by glutamate decarboxylase (GAD) immunohistochemistry. In the striatum (caudato-putamen, accumbens nucleus) a medium density of GAD-positive terminals was observed; a small number of medium-to-large size neurons and the vast majority of medium-size neurons were GAD immunoreactive. In addition, opioid peptide-like immunoreactivity was colocalized in a subclass of GAD-positive medium-size striatal neurons. The pallido-nigral system (GP, VP, EP, SNR) displayed a high density of GAD-positive axon terminals which synapsed upon dendrites and nerve cell bodies. The majority of pallido-nigral neurons also were GAD-immunoreactive. In contrast, the substantia nigra pars compacta and the subthalamic nucleus contained only few GAD-immunoreactive neurons.