Mice acclimatized to altitude (350 mm. Hg) showed an increase in performance when exposed to a higher altitude (247 mm. Hg) as compared with ground level controls. When acclimatized mice are bled they showed a decrease in performance level. This decrease in performance is not in proportion to the blood withdrawn, indicating that an increase in hemoglobin concn. is probably not the sole criterion for an increase in performance at altitude.