Elevated levels [in huam subjects] of inspired CO2 and blood PCO2 [CO2 partial pressure] resulted in moderate elevation of IOT [intraocular tension]. A marked rapid decrease in IOT to a level less than baseline value was noted when inspired CO2 was suddenly decreased to ambient levels. Decrease in IOT was more pronounced than the decrease in PCO2 and increase in blood pH. Changes in IOT appeared related to the rate of change of PCO2 rather than the actual level of PCO2. Increased ventilatory excursions with constant inspired CO2 levels did not cause any elevation of IOT, but a minimal compensatory drop in IOT below resting values occurred when increased ventilatory excursions were discontinued. The changes in IOT may be the result of sudden changes in aqueous production or ocular blood volume.