Serum lipid components were measured in eight male subjects exposed to an altitude of 14,000 feet for 14 days. Concentrations of total lipids rapidly decreased after the third day of exposure and a similar, although less pronounced decrease was observed in cholesterol levels. By way of contrast, phospholipid and FFA levels progressively increased during the period of altitude exposure. High altitude had no effect on serum glucose concentration but caused a slight increase in serum water content. The data indicate that high altitude has a marked effect on lipid metabolism.