In this study, radioimmunoassay (RIA) was performed to measure the change of immunoreactive-oxytocin (ir-OT) content in rat brain' nuclei related pain modulation, spinal cord, pituitary, adrenal gland and plasma by electroacupuncture "Zusanli" (EA) for 30rnin or pain stimulation for 2min. In the EA group, OT content was decreased greatly in the hypothalamic supraoptic N., while increased significantly in the hypothalamic ventromedial N., raphe magnus N., globus pallidus, spinal cord and adrenal gland, and there were no significant change in the hypothalamic paraventricular N., raphe dorsal N., locus ceruleus, pituitary arid plasma. There were also significant changes of OT content in rat brain, spinal cord and pituitary in the pain stimulation group, which are different from those of the EA group. The above results suggest that the changes of OT content in the EA group are specific, and provide evidence for involvement of endogenous OT in the central nervous system in acupuncture analgesia