Traditional folk medicine in the Arctic and Himalayan areas used Rhodiola species to enhance physical endurance, prevent aging, resist acute mountain sickness (AMS), and to treat fatigue, depression, anemia, impotence and respiratory infections. Rhodiola crenulata are widely used to prevent AMS in Himalayan areas and Lhasa in Tibet but none was examined by human study. The investigators conducted a randomized, double blind, placebo controlled, crossover study to investigate the efficacy of Rhodiola crenulata in preventing AMS.