Heat acclimatization is actually a sweat gland adaptation, and a 2-week process following arrival to a hot climate such as the tropics or a desert. Gradually sweat-evaporation is increased and the NaCl loss is reduced. The sweat secretion capacity may reach 4 l each hour with a thin sweat. The adaptation is caused by increased aldosterone secretion from the adrenal cortex. Aldosterone increases the reabsorption of NaCl and the secretion of K+ from the sweat, during its passage of the sweat gland tubules. The larger sweat loss the thirstier one feels. This is because the large sweat secretion reduces the time period for NaCl- reabsorption in the sweat gland tubules. The resulting high NaCl concentration in the plasma implies thirst, so heat adapted persons have to drink a lot. Thirst is an extremely late indicator of dehydration during work in a warm climate.