Thalamic nuclei implicated in the neural mechanisms of dyskinesia (1) have projections to components of the basal ganglia; (2) receive efferents from the corpus striatum, and/or (3) project fibers to regions of the cerebral cortex that generate signals which produce or modulate motor phenomenon. The neostriatum receives a major input from the intralaminar thalamic nuclei (ITN). Thalamostriate fibers projecting to the caudate nucleus (CN) and the putamen (Put) originate from different neuronal populations; clusters of cells in the rostral ITN and in the parafascicular nucleus (PF) project to the CN, while cells in the centromedian nucleus (CM) project to the Put. Smaller numbers of cells in the medial, dorsal and ventral nuclear subdivisions of the thalamus also project to the striatum. The amygdaloid nuclear complex receives afferents from the midline thalamic nuclei. Ventromedial and rostral parts of the subthalamic nucleus receive a small input from the centromedian-parafascicular nuclear complex. Segments of the globus pallidus (GP) and the substantia nigra (SN) do not receive afferents from either the cerebral cortex or the thalamus. Thalamic afferents originate ipsilaterally from the medial segment of the globus pallidus (MPS), and the pars reticulata of the substantia nigra (SNR), and contralaterally from the deep cerebellar nuclei (DCN). Each of these projections to nuclear subdivisions of the thalamus is distinctive without overlap. Projections from the MPS are to the ventral anterior pars principalis (VApc) and ventral lateralis, pars oralis (VLo) thalamic nuclei with collaterals to CM. The SNR provides projections to the ventral anterior, pars magnocellularis, the ventral lateral, pars medialis and the mediodorsal, pars paralaminaris thalamic nuclei. Output from the subthalamic nucleus (STN) projects to both the MPS and the SNR and could modulate influences upon thalamic nuclei. In the monkey the projection of STN to the GP is four times greater than to the SNR. The most massive input to the thalamus arises from the contralateral DCN and terminates in the so-called cell-sparse zone, which consists of the ventral posterolateral nucleus, pars oralis, the ventral lateral nucleus pars caudalis, and pars postrema and area x of Olszewski. Nuclear subdivisions of the thalamus receiving afferents from the MPS and the SNR have gamma-aminobutyric acid (GABA) as their major neurotransmitter; fiber systems originating from the DCN appear to have glutamate as their neurotransmitter.(ABSTRACT TRUNCATED AT 400 WORDS)