Central hemodynamic parameters were registered by right-side heart catheterization before and after intravenous administration of 12 mmol magnesium chloride (MgCl) in 15 patients with chronic ischemic heart disease and heart failure, New York Heart Association classes II and III. Serum magnesium concentrations increased from 0.76±0.03 (mean ± SD) to 1.54±0.05 mmol/1, which resulted in a reduction in mean arterial as well as pulmonary artery pressure by 10% (p<0.0001) and 7% (p<0.05), respectively. This reduction was caused by a marked decrease in systemic as well as pulmonary vascular resistance (from 1323 ±205 to 1132 ± 158 dyn•s/cm5, p<0.001 and from 156±73 to 133±72 dyn•s/cm5 (p<0.05). Heart rate, cardiac index, stroke volume index, and stroke work index increased slightly, although these differences did not reach statistical significance. Right and left ventricular filling pressures were not influenced, which indicates that the dilatory effect of magnesium, at the dosages used in the present study, is pronounced only at the arterial side of the vascular bed. The observed hemodynamic effects of the magnesium infusions may be beneficial in the setting of an acute myocardial infarction by reducing left ventricular afterload, which, together with the antiarrhythmic effect of magnesium may contribute to the positive effect of magnesium infusions on mortality in patients with acute myocardial infarction.