The following study was conducted to assess the biochemical and nutritional status of new military recruits during 12 weeks of strenuous physical activity. Calorie and magnesium (Mg) intake, energy expenditure, and serum, red blood cell (RBC) and mononuclear cell (MNC) Mg were assessed at the start, after 6 weeks and after 12 weeks of training. The results provide evidence that MNC Mg content decreases, whereas serum Mg increases, under prolonged, strenuous training conditions in previously unconditioned military recruits. Mg dietary intake alone could not account for these changes. It is postulated that this decrease in MNC Mg (from 64.76 +/- 34.99 to 23.81 +/- 15.55 fg/cell), unparalleled by similar changes in serum Mg or RBC Mg, reflects a reduction in exchangeable Mg body stores, and the onset of a Mg deficiency state.