The response to ACTH stimulation and to dexamethasone inhibition was studied in young adult males acutely exposed to an altitude of 14,000 feet. The results indicate that the adrenal cortex can be further stimulated with exogenous ACTH during the first day of exposure to an environment of low oxygen tension. Dexamethasone administration was started the day before the trip to high altitude and depressed the excretion of urinary steroids despite the hypoxic stimulus. © 1966, SAGE Publications. All rights reserved.