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Objectives: Clinical impact of magnesium (Mg) therapy remains controversial in acute myocardial infarction. We investigated the infarct size limiting effects of Mg and its mechanism in rabbits. Methods: Anesthetized rabbits underwent 30 min coronary occlusion and 3 h reperfusion in ten groups: (1) Control, (2) Low Mg, (3) Mg, (4) High Mg, (5) calcium (Ca), (6) Mg+Ca, (7) 8-phenyltheophylline (8PT), an adenosine receptor blockade, (8) 8PT+Mg, (9) α, β-methylene-adenosine diphosphate (AOPCP), a selective inhibitor of ecto-5′-nucleotidase, and (10) AOPCP+Mg groups. Infract size (IS) to area at risk (AR) was measured by triphenyltetrazorium chloride method. Results: The IS/AR ratio was significantly smaller in Mg, 27±3% (P<0.05) and High Mg, 24±2% (P<0.05) compared to Control, 50±3% and Low Mg, 42±4%. The IS limiting effects of Mg were abolished in 8PT+Mg, AOPCP+Mg and Mg+Ca. The IS/AR ratio correlated with neither rate-pressure products nor incidence of arrhythmia. Conclusion: Magnesium administration has an infarct size limiting effect independent of its effects on myocardial oxygen consumption and incidence of arrhythmia in rabbits. The infarct size limiting effect of magnesium is attributable, at least in part, to augmentation of adenosine mechanism. [Copyright &y& Elsevier]