Little is known about effects of naloxone, an opiate receptor antagonist, upon experimental autoimmune myocarditis (EAM) in mice. The aim of the present study was to test the effects of naloxone upon EAM in mice. Four-week-old C57BL/6 mice were injected with porcine cardiac myosin. Naloxone 1 mg/kg/day was given intraperitoneally daily on days 0 to 21 in experiment I (acute stage) and on days 21 to 42 in experiment II (chronic stage). For the analysis of endogenous opiate and neurohumoral system, plasma beta-endorphin and cytokines were measured. The cytotoxic activity of lymphocytes was determined by Cr-release assay. Naloxone reversed hypotension produced by massive myocardial injury in experiment I. In experiment I, the severity of cardiac pathology and inflammatory cytokines were decreased in the naloxone-treated group associated with higher beta-endorphin level compared with the untreated group. In addition, naloxone induced a shift from Th1 cytokine toward Th2 cytokine balance. Thus, cytotoxic activities of lymphocytes against EL-4 tumor cells were lower in the treated group than in the untreated group in experiment I, but not in experiment II. Naloxone is beneficial for heart failure caused by EAM in the acute stage, but not in the chronic stage, due to decreasing cytotoxic activities of lymphocytes and to its neurohumoral modulating effects.