||Seven volunteers from Salt Lake City (altitude 4,260 ft.) back-packed over a period of six days in Wyoming's Wind River Mountain Range between altitudes of 9,280 ft and 12,400 ft. Consecutive 24 hour urine aliquots were collected throughout the sojourn in addition to a 24 hour urine specimen previously collected in Salt Lake City. Urinary sodium (Na+), potassium (K+), and aldosterone levels were determined by laboratory assay after completion of the sojourn. Using 2-tailed paired T-tests, urinary sodium (Na+) and potassium (K+) levels did not change significantly during the sojourn. The sodium/ potassium ratio (Na+/K+) increased significantly on the fourth, fifth and sixth days (p ? 0.01, 0.01, 0.02, respectively). ' Aldosterone levels decreased significantly on day two (p 0.01) and on day five (p ? 0.02). Using the Pearson-R correlation coefficient, sodium was found to correlate directly to aldosterone throughout the sojourn with a coefficient value of 1.0. Normally exercise increases aldosterone production, however, in this study, the decrease in aldosterone may be explained by the inhibitory effect of hypoxia possibly overriding the effects of exercise. The correlation between aldosterone and sodium may be representative of aldosterone's control of sodium balance.