To the Editor.
—The questions raised by Drs. Rowland and Penn are both important and pertinent. Their case reports show that men who perform manual labor in hot weather and who develop muscle cramps may demonstrate myoglobinuria and an abnormal urinary sediment.In my statement (on p 850 of the review) that there have been no reports of myoglobinuria in patients with heat cramps, I refer to a form of environmental heat illness seen in those who perform hard physical work in the heat; the illness consists of profuse sweating, loss of salt and water, and replacement of water without salt, which lead to hyponatremia and muscle cramps. These individuals are often highly acclimatized to heat and highly conditioned for performance of hard physical work. Since physical conditioning generally protects against exertional muscle injury, one might suspect that cramps in such highly conditioned men would not lead to an elevation