Our previous studies have shown that 100Hz electroacupuncture (EA) produced antinociception through the release of endogenous opioids (mainly dynorphin) and the activated -opioid receptors in normal rats. Acupuncture is an effective treatment in relieving pain, but it develops tolerance after epeated administration. It has been reported that N-methyl-d-aspartate (NMDA) receptor antagonists could increase the antinociceptive effects induced by morphine and delay the development of tolerance to morphine but nothing has yet been described to reduce EA tolerance. Here we test whether ketamine, a non-competitive NMDA receptor antagonist, would enhance 100Hz EA antinociception as well as prevent or delay the development of chronic tolerance to 100Hz EA in normal rats. The results are as follows: (1) ketamine injected intraperitoneally (i.p.) 15min prior to EA enhanced the antinociceptive effects of 100 Hz EA at a dose of 5.0mg/kg, but not 0.2 or 1.0mg/kg. However, ketamine at either dose did not affect the basal nociceptive threshold (represented by tail-flick latency). (2) Ketamine at a dose of 5.0mg/kg delayed the development of chronic tolerance to 100Hz EA antinociception. We conclude that ketamine can enhance antinociception of 100Hz EA and delay the tolerance to 100Hz EA in rats. These results suggest that the development of 100Hz EA tolerance to antinociception was mediated, at least in part, through peripheral NMDA receptors, which may be useful in improving the therapeutic effects of EA in the treatment of pain when EA tolerance occurs.