Subjects breathing air at ground level or oxygen at a simulated altitude of 30,000 ft., who hyperventilated with the aid of a pneumolator until the alveolar pCO2 was lowered to about 18 mm. Hg, showed an avg. increase of about 20% in the magnitude of the corneo-retinal potential. Exposure to 20,000 ft. simulated altitude breathing air also increased the potential, but this result is attributed largely to the acapnia which normally accompanies such a procedure rather than to anoxia per se. In some individ- uals, however, anoxia itself seemed to increase the potentials.