The antagonistic effects of magnesium ion as a calcium ion blocker may decrease calcium influx associated with ischemia. However, the effect of magnesium on the preischemic neonatal myocardium has not been investigated previously. The purpose of this study was to investigate the effects of the administration of increasing doses of magnesium on left ventricular performance in the neonate. We assessed left ventricular function (pressure-volume data obtained by the conductance catheter/micromanometer technique) in three groups (n=6 in each) of newborn pigs (3 to 5 days old) differing with respect to magnesium concentrations. Endsystolic elastance did not change during infusion in group A (magnesium=1.2 mmol/L), whereas in groups B (magnesium= 8 mmol/L) and C (magnesium=16 mmol/L) it decreased significantly (P<.05) to 67±6% and 44±8% of baseline, respectively. The decrease in end-systolic elastance wasassociated with a significant reduction in cardiac output (P<.05) and stroke work (P<.05) in group C. After administration of magnesium, end-systolic elastance returned to baseline in group B in contrast to group C (78±3% of baseline value, P<.05). The slope constant of the end-diastolic pressure- volume relation decreased significantly (P<.05) from the preinfusion baseline values of 0.42±0.08 mL−1 in group B and 0.46±0.05 mL−1 in group C to 0.3±0.04 and 0.26±0.03 mL−1, respectively, versus no change in group A. At the end of the experiment, the slope constant returned to baseline value in group B, whereas in group C it remained significantly lower (P<.05, 78±3% of baseline). We conclude that administration of 8 to 16 mmol/L magnesium affects the systolic function and alters the diastolic properties of the neonatal heart in a dose-response manner.