Serial changes in serum magnesium (Mg2+) were assessed in unacclimatized males during treadmill exercise in a hot (49 degrees/27 degrees C, dry/wet bulb) environment. Eight subjects walked at 5.6 km/h on a 0% grade, and at 90 min of exercise had a mean heart rate of increase of 70 plus or minus 5 (S.E.) beats/min, rise in rectal temperature of 2.1 degrees plus or minus 0.1 degrees C, and a weight loss of 2.07 plus or minus 0.10 kg. There was a significant (p less than 0.01) decrease in mean serum Mg2+ concentration (control: 1.87 plus or minus 0.06 mEq/I; 45 min: 1.81 plus or minus 0.07; 90 min: 1.72 plus or minus 0.08). The Mg2+ concentration in sweat samples collected over 90 min of work in the heat averaged 0.28 mEq/I (range 0.13 to 0.45). Thus, in normal man, exercise in an hot, dry environment resulted in a fall in serum magnesium which was not wholly explicable on the basis of sweat loss of this ion.