Current methods of assessing cerebral blood flow (CBF) are limited in their ability to provide data at the bedside in a timely, inexpensive, and continuous fashion. Since the palpebral conjunctiva is perfused by branches of the internal carotid artery, perfusion of this tissue may reflect global CBF. Conjunctival oxygen tension (Pcjo2). Pao2, Paco2, and pH were measured in ten healthy subjects during normal ventilation and active hyperventilation. CBF was measured simultaneously using positron emission tomography. CBF decreased from an average of 64.3 ± 15.1 ml × 100 g−1× min−1during baseline measurements to 33.2 ± 8.4 ml × 100 g−1× min−1during hyperventilation. The ratio of Pcjo2to Pao2(the Pcjo2/Pao2index) decreased from 0.53 ± 0.07 to 0.35 ± 0.09 in the same time period. The Pcjo2/Pao2index was significantly correlated with CBF (r = .78,p< .001). We conclude that the Pcjo2/Pao2index may reflect the reduction in CBF induced by hyperventilation in normal humans, and should be investigated further as a method of assessing CBF in other settings which can result in globally reduced cerebral perfusion.