The effects of electrical stimulation of the nucleus tegmenti pedunculopontinus on the unitary activity of identified neurons of the rat substantia nigra were studied. The experiments were carried out in intact rats as well as in animals bearing either chronic bilateral electrolytic lesions of the deep cerebellar nuclei or an acute lesion of the ipsilateral subthalamic nucleus. Excitation of both compacta and reticulata cells of the substantia nigra (many of the latter being output neurons since they are antidromically activated from the superior colliculus) was the predominant response recorded. Two types of excitations could be distinguished. The first was a direct orthodromic excitation (latency 2.9 +/- 1.6 ms; duration 3.7 +/- 1.9 ms). The second was a sparse and less pronounced activation (latency 5.2 +/- 1.8 ms; duration 13.0 +/- 3.0 ms). These two types of excitation were the only responses recorded in intact rats (10/51, 19.6%, orthodromic and 10/51, 19.6%, diffuse activation). When the cerebellar nuclei were destroyed 7-21 days prior to the recording, both excitations were still found (10/59, 16.9% and 15/59, 25.4%, respectively), whereas a minority (3/59, 5.0%) of neurons were inhibited. Conversely, when the subthalamic nucleus was lesioned the orthodromic response was still present (9/42, 21.4%) whereas the occurrence of the diffuse excitation greatly decreased (3/42, 7.1%) and a greater number of inhibitions (6/42, 14.2%) appeared. A small population of cells (12/85, 14.1%) were excited from the contralateral pedunculopontine nucleus either by the orthodromic or by the diffuse excitation. The total number of nigral neurons antidromically activated from the ipsilateral pedunculopontine nucleus was 9/152 (5.9%). The results provide evidence that the nucleus tegmenti pedunculopontinus gives a dual excitatory input to the substantia nigra either through a probable direct connection or through a polysynaptic pathway via the subthalamic nucleus. A few cells from both parts of the substantia nigra, in turn, project back to the nucleus tegmenti pedunculopontinus. In addition, our data give further support to the view that output fibers from the deep cerebellar nuclei do not synapse in the substantia nigra in the rat.