An electrode was implanted to stimulate unilaterally the mesencephalic ventromedial tegmentum (VMT) in 27 adult cats. Abnormal involuntary movements (AIM) such as contraversive head turning, bucco-lingual movements and choreoathetoid movements of the contralateral forelimb were elicited by high frequency electrical stimulation (20-100 Hz); the cats were unrestrained. Such AIM were abolished by damaging the thalamic ventrolateral nucleus ipsilateral to the stimulated side. In order to investigate the neural pathway related to AIM, kainic acid (2μg) was injected stereotaxically into either the caudate nucleus or the globus pallidus in cats which had AIM during electrical stimulation of the VMT. These experimentally produced AIM did not changed with injection of kainic acid into the ipsilateral anteroventral part of the caudate uncleus, but were abolished by injection either into the ipsilateral rostromedial part of the caudate nucleus or into the globus pallidus. It was confirmed histologically that the intracerebral microinjection of kainic acid produced degeneration of nerve cell bodies and dendrites near the injection site while not affecting axons terminating in or passing through it. These results suggest that the neural pathway arising from the substantia nigra and leading to the ventrolateral thalamus via the rostromedial part of the caudate nucleus and the globus pallidus are essential in producing AIM by electrical stimulation of VMT, and that this neural pathway contains at least one synapse each within the caudate nucleus and the globus pallidus.