Kester''s theory that the tropical sun causes an increase in the blood count is supported, and by the rays substances are formed in the air which stimulate the formation of red corpuscles if the rays are inhaled. The effective factor in high altitudes is not decrease in 0 but more intense irradiation of the sun''s rays. The blood count of the author and 2 companions largely increased with altitude and was greater than that of other subjects taken on Mt. Everest, which lies outside the tropics. The party had more nausea when at rest than when walking. The 2 classic symptoms of mountain sickness, nausea and cephalalgia, were experienced without an essential manifestation of O want being present. Moderate uphill walking produced no marked palpitation or breathlessness and mountain sickness was stopped by walking. Further tests showed that expiratory force does not vary with the altitude.