The motor symptoms of Parkinson's disease (PD) are thought to result from increased inhibitory outflow from the basal ganglia to the pallidal receiving areas of thalamus (ventralis oralis anterior and posterior -Voa,Vop). To test this hypothesis, we examined the firing rates of neurons in pallidal and cerebellar receiving areas of thalamus in 5 PD patients and compared them to those of neurons in comparable regions of motor thalamus in 2 other patient groups where hyperactivity of GPi is not believed to occur (essential tremor (ET), pain). Neuronal recordings were made during microelectrode guided functional stereotactic neurosurgery. The mean spontaneous firing rate (MSFR) of neurons classified as voluntary neurons and presumed to be in pallidal receiving areas of thalamus in PD patients (7.4±1.0 Hz, mean ± SEM) was significantly lower (p < 0.01) than in the ET (18.1±3.0 Hz) and Pain (19.0±1.9Hz) groups. In contrast, the MSFR of neurons classified as kinesthetic and presumed to be primarily in the cerebellar receiving area of thalamus (ventral intermediate -Vim),although some are probably in the deep shell region of the ventrocaudal nucleus (VPLa), was significantly greater in ET patients (25.8±3.5 Hz) than in the PD (14.3±1.6 Hz) (p < 0.01) and Pain (16.1±1.5 Hz) (p < 0.05) groups. Similar findings were obtained when the neurons were grouped according to their estimated locations in Voa/Vop and Vim of motor thalamus. These data provide support for the prediction of the classical pathophysiological model of PD and moreover suggest that pathophysiology in the cerebello-thalamo-cortical pathway may be a possible cause of tremor in ET patients.