We examined whether healthy young swimmers improved performance during emotional and physical stress testing in the laboratory or during competition. Twenty four well trained healthy Caucasian pupils (14 boys and 10 girls, means = 16 years) were randomly treated either with 20 mmol Mg-aspartate-HCl/d or placebo for 3 months. After Mg supplementation serum Mg increased from 0.85 +/- 0.03 to 0.89 +/- 0.06 mmol/litre (P less than 0.05). Mg in red blood cells remained unchanged (4.79 +/- 0.35 mmol/kg dry weight before supplementation and 4.96 +/- 0.27 three months later, P = 0.32) whereas no increase in serum Mg or Mg in red blood cells was found in subjects who received placebo. Student's t test and analyses of variance revealed no significant differences in performance data either in the laboratory or during competition after supplementation with Mg.