A polarographic conjunctival oxygen sensor was used to measure oxygen tension in a tissue bed supplied by the internal carotid artery. The shared vascular source of the conjunctiva and brain suggests that conjunctival Po2monitoring may provide an index of cerebral perfusion. We studied the effects of hyperventilation, a known stimulus of cerebral vasoconstriction, on conjunctival oxygen tension (PcjO2) in six normal, healthy adults; arterial blood gases were simultaneously measured in four of these subjects. A 5-min period of hyperventilation to a Paco2near 20 torr resulted in a rapid and significant (p< .01) increase in systemic oxygen tension as measured by arterial blood gases and a transcutaneous oxygen monitor. These values gradually returned to baseline upon cessation of hyperventilation. PcjO2, near however, decreased significantly (p< .01) during hyperventilation, suggesting vasoconstriction of the conjunctival vascular supply. Because these changes temporally correlate with the cerebral vasoconstriction during hyperventilation, the conjunctival index of tissue oxygen tension may correlate with cerebral perfusion.