The endogenous analgesic system may be connected to the development of chronic postoperative pain. The terms “diffuse noxious inhibitory controls” (DNIC; in animals) or “conditioned pain modulation” (CPM; in human beings), refer to a “pain inhibits pain” phenomenon whereby the activities of convergent neurons [wide dynamic range neurons (WDR neurons)] in the spinal dorsal horn and trigeminal nucleus are selectively and powerfully inhibited by the application of a stimuli to another body area distant from their excitatory receptive fields. DNIC and CPM involve a supraspinal loop, including the serotonergic and noradrenergic systems. The relationship between postoperative pain and CPM effect has been reported. Individual response to CPM can vary widely among patients. Therefore, the risk for chronic postoperative pain may be predicted by using CPM tests before surgery. In this review, we discuss the neuronal mechanisms of DNIC and CPM, the methodology of CPM, and the potentiality of CPM as a screening test for chronic postoperative pain.