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The purpose of this study was to determine whether the pressor effect of infused calcium is inhibited by increased serum magnesium.Intravenous infusions of calcium, magnesium, and calcium + magnesium were performed in 10 subjects and the effect upon haemodynamic variables was investigated.Intra-arterial blood pressure and heart rate were recorded during i.v. infusions of magnesium, calcium and calcium + magnesium. Cardiac output and serum concentration of electrolytes were measured before and after the infusions. Total peripheral vascular resistance (TPVR) was calculated from: mean blood pressure/cardiac output.Magnesium infusion increased serum magnesium, but did not change blood pressure, cardiac output, or TPVR. Calcium infusion increased serum calcium and elevated both blood pressure and TPVR with no change in cardiac output. Calcium + magnesium infusion increased both serum magnesium and calcium, showing elevation of blood pressure and TPVR with no change in cardiac output. Changes in haemodynamic variables caused by the calcium + magnesium infusion were similar to those caused by the calcium infusion alone.These results suggest that increased serum calcium elevates blood pressure and TPVR through arterial vasoconstriction by arterial smooth muscle contraction, but increased serum magnesium does not induce arterial vasodilation.