Plasma arginine vasopressin levels were studied in 15 normal working men exposed at 4-day intervals to a rapid increase or decrease in altitude of 2000 m. Plasma arginine vasopressin levels were significantly lower (P less than 0.001) at the higher altitude though plasma osmolality did not change. It is concluded that the important diuresis known to physiologically occur in response to high altitude may be related to a decrease in antidiuretic hormone release.