Vibratome sections of rat substantia nigra (SN) topically injected with colchicine were processed for glutamate decarboxylase (GAD) immunocytochemistry to reveal GABAergic neurons using electronmicroscopic procedures. Both, GAD-immunoreactive neurons and non-immunoreactive neurons receive a dense innervation of GAD-immunoreactive nerve terminals. In an attempt to clarify the origin(s) of this GABAergic input, the projections between caudate-putamen (CP) and SN were lesioned by circumscribed ibotenic acid injections in CP, or by complete hemitransection either at the level of the globus pallidus or at the frontal pole of SN. In addition, the axonal transport was blocked by local injection of colchicine. After survival times from 40 h to 7 days, the interrelations of GAD-immunoreactive neurons and of unstained neurons with degenerated and preserved boutons were investigated. Striatal terminals contact GAD-negative (presumably dopaminergic) neurons, and, at least as frequently, the GABAergic pars reticulata neurons. Numerous GAD-immunoreactive boutons are apparently intact after the different types of lesions; also the spared GABAergic boutons synapse on both, GAD-positive and GAD-negative neurons. Thus, at the level of SN the striatonigrostriatal loop as well as the striatonigrothalamic (-tectal) projections are under the control of inhibitory axon collaterals of the GABAergic pars reticulata neurons.