The electrolyte composition of sweat secreted in response to a standard mild thermal stimulus was found to be abnormal in 43 patients with cystic fibrosis of the pancreas. Sodium and chloride concentrations in sweat were 2 to 4 times as high as those found in 50 patients with a variety of other diseases. Sweat potassium levels were also increased in cystic fibrosis of the pancreas although not to the same extent. In contrast, the amount of sweat was not significantly different in patients with fibrocystic disease of the pancreas and control patients. It was shown that pancreatic deficiency, chronic pulmonary disease and the administration of certain drugs do not per se affect the electrolyte composition of sweat. It was also demonstrated by metabolic studies that disturbances in renal and adrenal function are not responsible for the high sweat chloride and sodium in fibrocystic disease. The dissociation between increased electrolytes in the sweat and their very low levels in the urine during salt restriction are unique to this condition. The fact that the sweat glands do not retain salt effectively in hot weather, during salt restriction or when DCA is administered, points to a disturbance in the sweat glands themselves. This abnormality may not be pathognomonic of cystic fibrosis of the pancreas, but it is an expected finding. These disturbances in sweat composition together with preliminary observations of abnormal salivary gland secretion suggest that the secretory activity of many and perhaps all exocrine glands is affected in cystic fibrosis of the pancreas. The term "mucoviscidosis" does not seem justified in view of the fact that at least two nonmucus-secreting structures are affected in this condition. Massive salt loss through sweating in hot weather accounts satisfactorily for the "heat casualties" in cystic fibrosis of the pancreas. The mechanisms are discussed and it is emphasized that the symptoms are initiated by pure salt loss and the consequent reduction in extracellular fluid volume. Since the condition may, if not treated, lead to fatal cardiovascular collapse, it should be regarded as an acute medical emergency. Under the usual circumstances, serum electrolyte concentrations are normal in fibrocystic disease of the pancreas. In the presence of marked pulmonary dysfunction in this disease, the bicarbonate of the plasma is elevated and the chloride reduced in compensation. It is therefore necessary to demonstrate a lowered serum sodium as well as chloride to detect the massive salt loss which may occur in hot weather.