Objective Accumulated data show that the mechanisms of transcutaneous electrical acupoint stimulation (TEAS) analgesia involve the activation of endogenous opioid antinociceptive systems and the μ-opioid receptors (MORs) in the central nervous system (CNS).Positronemission tomography (PET) is the only real time in vivo method available today to study μ-opioid receptor function.In this study we investigated the different effects of low and high frequency TEAS on MOR binding potential of the brains in rhesus monkeys.Methods PET with 1 1C-carfentanil was performed three times randomly under TEAS of 0 Hz, 2 Hz or 100 Hz respectively in each monkey separated by at least 2 weeks.TEAS was administered in the middle 30 min during the 95 min PET scan.PET images were reconstructed in 3D OSEM mode.The distribution volume of three stages (before, during and after TEAS) were calculated using Logan plot with an online arterial radiocounting as input function and were analyzed using Statistical Parametric Mapping (SPM).Results 2 Hz TEAS evoked an increases in MOR binding potential in multiple pain and sensory processing brain regions including the dorsal cingulated cortex, caudate nucleus, putamen, temporal lobe, somatosensory cortex and amygdale during the TEAS compared with that of 0 Hz TEAS.The effect remained after TEAS in genual anterior cingulate cortex and temporal lobe.No increase in MOR binding potential was found during and after the 100 Hz TEAS compared with 0 Hz TEAS.Conclusion The increases of μ-opioid receptor binding potential in multiple pain and sensory processing brain regions may play a role in mediating the effect of low, but not high frequency TEAS.