Experimental time course was as follows: 20 min working perfusion, 3 min cardioplegic infusion with St Thomas Cardioplegic Solution followed by global ischemia for 35 min at 37.5 degrees C, 15 min first Langendorff reperfusion with reperfusion solution (RS) with various concentrations of Mg and 5 min second reperfusion with KHBB, followed by 20 min working. Cardiac functions were measured during pre and post working perfusion and CK leakage were measured during reperfusion. Percent recoveries of aortic flow at the Mg concentration of 0, 0.6, 1.2, 3.0, 6.0, 12 mM were 21 +/- 5, 22 +/- 3, 48 +/- 2, 37 +/- 4, 28 +/- 3, 15 +/- 3 (%) (mean +/- SEM), respectively. Our data indicated that 1.2 mM Mg of RS possessed protective properties with bell shaped dose response characteristics.