The changes in intraocular pressure due to blood pH, PCO2, and PO2 alterations induced by hyperventilation and hypercapnea in man undergoing routine general anaesthesia were studied. It was shown that hypercapnea produces elevation of intraocular pressure, while hyperventilation lowers it. Acetazolamide pretreatment did not alter these IOP responses to PCO2, PO2, and blood pH changes.