The cellular origin and degree of collateralization of the pallidointralaminar and pallidonigral projections in squirrel monkey (Saimiri sciureus) were studied using Evans blue (EB) and a mixture of DAPI-Primuline (DP) as fluorescent retrograde tracers. In a first series of experiments EB was injected into the VA/VL thalamic nuclei whereas DP was delivered into the CM/Pf complex. After these injections numerous EB-labeled cells were scattered throughout the central 'motor' zone of the internal segment of the globus pallidus (GPi), compared to a smaller number of DP-positive neurons forming two small but distinct clusters in central GPi. The majority of the neurons within these two clusters were double-labeled. In addition, EB-labeled cells were disclosed in the lateral two-thirds of substantia nigra pars reticulata (SNr), whereas DP-positive neurons occurred in a wide variety of structures including the nucleus reticularis thalami, the SNr, the periaqueductal gray, the superior colliculus, the midbrain raphe nuclei, the pedunculopontine nucleus (TPP) area (bilaterally), and the locus coeruleus. In a second series of experiments, EB was injected into the CM/Pf complex whereas DP was delivered into the TPP area of the midbrain tegmentum. After these injections two small clusters of EB-labeled cells and a larger number of more uniformly distributed DP-positive cells occurred in the core of GPi. The cell clusters were similar in size and location to those observed after VA/VL-CM/Pf injection, but contained only a minority of double-labeled neurons. The distribution of non-pallidal cells projecting to CM/Pf complex was similar to that reported above, whereas retrogradely labeled cells resulting from TPP injection were disclosed in the paraventricular hypothalamic nucleus, the central amygdaloid nucleus, the preoptico-hypothalamic complex, the lateral habenula, the ventral tegmental area, and the SNr where some double-labeled cells were present. In a third series of experiments DP alone was injected into the entire substantia nigra (SN), involving both pars compacta and pars reticulata. The SN injection produced retrograde cell labeling in numerous structures such as the striatum, the rostral intralaminar nuclei, the subthalamic nucleus, the TPP area (bilaterally), the dorsal raphe nucleus and the locus coeruleus. At pallidal levels, a moderate number of DP-labeled cells occurred within the dorsal half of the external segment of the globus pallidus (GPe), whereas the GPi was virtually devoid of labeled neurons. The GPi appeared nevertheless surrounded ventromedially by numerous large-sized DP-positive cells belonging to the substantia innominata.(ABSTRACT TRUNCATED AT 400 WORDS)