The Anglo-American Pike's Peak Expedition, 1911, of which we propose to give the scientific results, was planned by members of Oxford and Yale Universities, with the main object of making a thorough study of physiological adaptation to low atmospheric pressures. It was decided to make all the observations on Man, as the conditions of respiration and the blood changes can be studied more satisfactorily in man than in animals. It was also essential that the physiological conditions, apart from the reduced atmospheric pressure, should be normal as far as possible, and that the observations should be continued over a considerable period. After considering the advantages and disadvantages of making the experiments in the Alps, Andes, or Himalayas, we selected Pike’s Peak, Colorado, as it seemed preferable in several very important respects. The Peak is practically free from snow in sum m er: there is a substantial house on the summit, in which we were offered excellent accommodation; and a cogwheel railway which ascends the mountain afforded easy transport for apparatus and supplies, and communication with the scientific laboratories at Colorado College, Colorado Springs, only about 14 miles distant. A further advantage was that large numbers of people came up daily during the summer, and furnished ample opportunities for observing the symptoms of mountain sickness in unacclimatised persons, while several persons living on the summit, besides ourselves, were available for studying the effects of acclimatisation.