Determinations of the amount of sugar in the blood by the revised Folin-Wu method showed that the blood sugar increased only a little as the subjects went higher up to an altitude of 6,140 m. in the Chilean Andes. It was doubtful how far this increase could be attributed to anoxemia. The blood sugar in work varied in approximately the same way at all altitudes and was related to the amount of work done in comparison to the total amount of work of which the individual was capable at a given altitude rather than to the absolute amount of work. The glucose tolerance was greatly increased in 2 subjects by 18 days at 5,340 m. and changed, though not increased, in a 3d subject by 6 days at 5,340 m.