The objective was to study effects of dietary Mg on acid-base status and Ca metabolism of Holstein cows fed acidogenic diets with relatively high Ca concentrations. Eight nonlactating, nonpregnant Holstein cows were used in a switchback experiment with three 28-d periods. The normal Mg (.2%, dry basis) diet consisted of corn silage plus a concentrate mix supplemented with NH4Cl (126 g/d per cow) and (NH4)2SO4 (126 g/d per cow). The high Mg (.37%, dry basis) diet had MgSO4 substituted for an equivalent amount of S supplied by (NH4)2SO4 in the normal Mg diet. Cation-anion differences of the two diets were -302 (normal Mg) and -289 (high Mg) meq/kg of dietary DM. Compared with cows fed the normal Mg diet, those fed high Mg tended to have higher blood pH and plasma concentrations of total Ca but lower plasma concentrations of P and lower urinary excretion of ammonium and net acid. Cows fed the high Mg diet also tended to increase Mg excretion with a decrease in urinary excretion of Ca. Metabolic responses to intravenous infusion of Na2-EDTA were similar among cows fed either diet. Results indicate that increasing Mg intake of cows fed acidogenic salts was of no advantage with regard to Ca metabolism.