The authors examined the effect of strenuous exercise on the serum lithium levels of four healthy, conditioned athletes who were stabilized on lithium carbonate for 7 days and who ran a 20-km race under hot, humid conditions. The subjects became substantially dehydrated during the race, and their serum lithium levels decreased, suggesting that sweat lithium loss may be substantial. (The sweat-to-serum ratio for lithium exceeded that for sodium by a factor of 4.) The authors conclude that contrary to widely held belief, heavy sweating may not increase the risk of lithium intoxication.