Hinchcliff, K. W., G. A. Reinhart, J. R. Burr, R. A. Swenson. Exercise-associated hyponatremia in Alaskan sled dogs: urinary and hormonal responses. J. Appl. Physiol. 83(3): 824-829, 1997.Exercise-associated hyponatremia occurs in horses and humans, both species that sweat, and in sled dogs, which do not sweat. To investigate the mechanism of exercise-associated hyponatremia in sled dogs, we measured water turnover, serum electrolyte concentrations and osmolality, plasma renal hormone concentrations, and urine composition of 12 fit Alaskan sled dogs before, during, and after a 490-km sled dog race (Ex group). Water turnover and serum electrolyte concentrations were measured in six similarly fit dogs that did not run (Sed group). Water turnover was significantly larger (P < 0.001) in Ex [190 ± 19 (SD) ml · kg¹ · day¹] than in Sed dogs (51 ± 13 ml · kg¹ · day¹). There were significant (P < 0.001) decreases in serum sodium concentration (from 148.6 ± 2.8 to 139.7 ± 1.9 mmol/l) and osmolality (from 306 ± 9 to 296 ± 5 mosmol/kgH₂O) of Ex, but not Sed, dogs during the race. Plasma concentrations of arginine vasopressin decreased, whereas aldosterone and plasma renin activity increased significantly (P < 0.01) during the race. Urine osmolality was unchanged, whereas urine sodium, potassium, and chloride concentrations decreased significantly (P < 0.05) and urine urea concentration increased (P = 0.06). These results demonstrate increased water turnover associated with hyponatremia and renal sodium conservation with maintained high urine osmolality in exercising Alaskan sled dogs.