In 26 top sportsmen (volleyball players, rower) aged 15-18 years the serum magnesium level and urinary Mg excretion before and after oral administration of 5 g Mg lactate was assessed (oral magnesium load test). In 11 sportsmen (42.3%) a low percentage of excretion of administered Mg was found, suggesting magnesium deficiency, irrespective of the basal serum Mg levels. After subsequent 10-day Mg supplementation 6 volleyball players responded by a markedly lower retention after an identical Mg load, again without marked changes of the serum Mg level. The authors consider the magnesium load test a more sensitive indicator of latent Mg deficiency than the serum Mg level which is maintained at a relatively stable level and declines only in severe deficiency. The test assumes normal renal function and intestinal absorption.