In some parts of the world, acupuncture has been employed as a method of obtaining analgesia for thousands of years without the mechanism of its action being understood. During the past two decades, evidence has accumulated indicating that acupuncture activates an intrinsic neural network which monitors and modifies the activity of pain-transmitting neurons. The in-suppressing action is partly mediated by endogenous opioid peptides and monoamines. The system is organized at three levels of the neuroaxis: spinal cord, medulla and the midbrain. The raphe magnus nucleus and the spinal cord constitute a fundamental circuit while the PAG funnels the influences from the more rostral structures and collects information from the spinal cord. PAG initiates descending and ascending nihibition resulting in the reduction of pain. The endogenous pain-control system may be elicited by other physiological stimuli and the effect of acupuncture is scarely specific.