Stimulation of the central gray matter areas has been used for the treatment of chronic pain for decades. To better understand the mechanism of action of such treatment we studied the effects of stimulation of the periventricular gray (PVG) on the sensory thalamus in two patients with chronic central pain. In each case, two electrodes were implanted in the PVG (Medtronic 3389) and the ventroposterolateral thalamic nucleus (Medtronic 3387), respectively, under guidance of CT/MRI image fusion. The PVG was stimulated in the frequency range of 2-100 Hz in alert patients while pain was assessed using the McGill-Melzack visual analogue scale. In addition, local field potentials (FPs) were recorded from the sensory thalamus during PVG stimulation. Maximum pain relief was obtained with 5-25 Hz stimulation while 50-100 Hz made the pain worse. This suggests that pain suppression was frequency dependent. Interestingly, we detected low frequency FPs at 0.2-0.4 Hz closely associated with the pain. During 5-25 Hz PVG stimulation the amplitude of this potential was significantly reduced and this was associated with marked pain relief. At the higher frequencies (50-100 Hz) however, there was no reduction in the FPs and no pain suppression. We have found an interesting correlation between thalamic activity and chronic pain. This curious low frequency potential may provide an objective index for quantifying chronic pain, and may hold further clues to the mechanism of action of PVG stimulation.