Supplemental administration of magnesium very early after the onset of acute myocardial infarction is supported by abundant data indicating potential cardioprotective effects of magnesium. Magnesium is considered to be "nature's physiologic calcium blocker." This is because it protects myocytes against calcium overload by inhibiting calcium influx which is particularly important at the time of reperfusion. In experimental models of ischemia and reperfusion, agents inhibiting calcium influx improved post-ischemic recovery of mechanical function when given prior to or at the time of reperfusion. On the other hand, little improvement in mechanical function was observed if such agents were given 15-20 minutes after the onset of reperfusion. Reduced serum magnesium may also be responsible for a maladaptive increase in coronary tone and an increased response to vasoconstrictors.