In order to investigate the effect of epidural morphine and electroacupuncture (EA) on immune function of postoperative patients, peripheral blood for NK (natural killer) cell activity assay were obtained from patients before and on the 1st, 3rd, 7th day after cholecystectomy. 1 mg morphine injected into epidural space after surgery significantly suppressed NK cell activities via prolonged inhibition and increased the inhibitory intensity of it. EA of "Zusanli (St. 36), Sanyinjiao(Sp. 6), Hegu(L.I.4) and Neiguan(P.6) points enhanced the NK cell activities after epidural injection of morphine. It was further observed that microinjection of 1 microgram/1 microliter naloxone into periaqueductal gray (PAG) partially antagonized the suppression of NK cell activity induced by intrathecal (ith) injection of 40 micrograms/50 microliters morphine, and so does EA of "Zusanli(St. 36)". When EA applied after naloxone microinjected into PAG, the NK cell activity was further augmented as compared to that using EA or PAG microinjection of naloxone alone. The above results suggest epidural morphine combined with EA is a better method to relieve postoperative pain, and opiate receptor in PAG might partially mediate the suppression of NK cell activity induced by intrathecal injection of morphine. Yet, the mechanisms of the attenuation of EA on morphine induced immunosuppression needs further study.