The projections of the substantia nigra pars compacta (SNc) to the reticular thalamic nucleus (RTn) were assessed by measuring dopamine content and counting tyrosine hydroxylase positive (TH (+)) cells in rats with unilateral lesions induced by 6-hydroxydopamine (6-OHDA), and by using a fluorescent tract-tracing technique in rats without lesions. Injection of 6-OHDA in the RTn reduced dopamine content and the number of TH (+) cells in the SNc by about 50%. Branching of SNc was suggested by the finding that 6-OHDA deposited in the RTn significantly reduced dopamine in the striatum and globus pallidus. Moreover, injections of 6-OHDA into either the striatum or the globus pallidus significantly reduced dopamine content in the RTn. Fluorescent tracers injected into the RTn labeled TH (+) cells in the SNc. A high proportion of these TH (+) cells was double labeled when tracers were also injected into either the globus pallidus or striatum. Other experiments showed that systemic injection of apomorphine or methamphetamine induced turning behavior in rats with local deposits of 6-OHDA in either the RTn or the studied basal ganglia nuclei. The extensive dopaminergic branching suggests that the abnormal motor behavior of rats with 6-OHDA deposits in the RTn may be caused by dopaminergic denervation of more than one structure. The fact that lesion of a single dopaminergic neuron can reduce dopamine transmission in more than one structure is probably important in generating the manifestations of Parkinson's disease.