Isolated hearts from rats subjected to high and low dietary Mg for 4 weeks were perfused with buffers containing 0.8 and 0.3 mM Mg, these values reflecting the plasma Mg concentrations on the two intakes. After a 20 min period of subtotal, global ischemia the recovery values of cardiac function were significantly lower in hearts from Mg-depleted rats perfused with 0.3 mM Mg, whereas the control values of cardiac function of the two groups were nearly identical before the ischemic episode. An additional group of hearts from Mg depleted rats perfused with 0.8 mM Mg did not present higher values of post-ischemic cardiac function compared to those perfused with 0.3 mM Mg. Rats fed a low dietary Mg had lower concentrations of Mg in plasma, bone and skeletal muscle, but not in the heart. The only myocardial electrolyte difference was a higher Na in the low dietary Mg group. It is concluded that an acquired extracellular Mg deficiency may reduce the myocardial tolerance to ischemia. The rapid restoration to high extracellular Mg in a prolonged Mg deficiency state did not improve the myocardial tolerance to an immediate ischemic episode.