Successive electro-acupuncture (EA) stimulation applied to bilateral anterior tibial muscles, where Zusanli (ST36) acupoints are located, once a day (30 min) for 3 successive days significantly enhanced splenic natural killer (NK) cell activity in BALB/c mice. The percentage of splenic NK cells, as measured by flow cytometry, was not affected in these mice. Interferon (IFN)-gamma level in splenic aqueous extract, prepared from the ST36 acupoint-stimulated mice, was significantly higher than that of the controls. In vivo treatment with neutralizing monoclonal antibody against mouse IFN-gamma completely abrogated the increase in splenic NK cell activity induced by ST36 acupoint stimulation. The same stimulation also significantly increased the concentration of splenic beta-endorphin, which coincided with the significant increase in splenic IFN-gamma production. Pre-administration of 10 mg/kg naloxone before initiation of EA stimulation every day reduced the enhancements of NK cell activity and IFN-gamma level. These observations strongly suggest that endogenous IFN-gamma mediates the up-regulation of NK cell activity by EA stimulation at the ST36 acupoints. Furthermore, endogenous beta-endorphin secreted by EA stimulation also plays an important role in the up-regulation of NK cell function, which may be realized through regulating IFN-gamma production.