OBJECTIVE: To investigate the effect of monophosphoryl lipid A (MLA) on ischemia/reperfusion (I/R) injury in rabbit heart and its signal transduction pathway. METHODS: Eighteen rabbits were randomly assigned to three groups: (1) MLA-group: Six rabbits received injection of MLA (35 &mgr;g/kg, iv) 24 h prior to 45 min of ischemia followed by 60 min of reperfusion. (2) Control group: Six rabbits received an intravenous bolus injection of the same volume of vehicle. (3) SB group: Six rabbits intravenously received 2 &mgr;mol/l SB 203580 30-min prior to MLA administration. At the end of reperfusion, myocardial infarct size and serum nitrite/nitrate were detected. Myocardium p38 mitogen-activated protein kinase (p38 MAPK) was detected using a Western blotting method. RESULTS: MLA pretreatment caused a significant reduction of infarct size as percent of area at risk as compared to vehicle-pretreated controls (36+/-3 vs. 65+/-4%, P<0.001). Total nitrite in serum increased in MLA pretreated animals (P<0.05). The protective effects of MLA were completely abolished by selective p38 MAPK inhibitor SB 203580. Western blot analysis showed significant accumulations of p38 MAPK proteins in the myocardium of MLA pretreated animals. CONCLUSIONS: MLA can protect the heart by reducing myocardial infarct size in rabbits. This cardioprotective effect might be attributed to upregulation of nitric oxide (NO) through the activation of p38 MAPK.