Plasma aldosterone, cortisol, sodium (Na), potassium (K), calcium (Ca), magnesium (Mg) as well as urine and sweat Na, K, Ca and Mg concentrations were measured in nine male healthy persons during an one hour ergometer exercise before and after a fourteen day magnesium aspartate (Mg) supplementation. The usual aldosterone and cortisol increase during exercise was not observed and cortisol concentration was significantly lower after Mg supplementation. Na and K in plasma increased during the exercise; these changes were not affected by Mg. The Mg concentration was elevated in plasma and erythrocytes after Mg supplementation. During the ergometer course plasma Mg was unchanged but decreased significantly in the red blood cells. Mg and K concentration in sweat decreased during the exercise. No influence of Mg on urinary electrolyte excretion was observed.