Oxygen-sensitive microelectrodes were used to measure oxygen profiles from the prelaminar region of the cat optic nerve head. From the profiles, the oxygen distribution of this tissue was determined. Changes in oxygen levels in response to flicker stimulation were obtained. Also, the regulatory ability of this tissue was assessed by acutely increasing IOP and by presenting the cat with periods of hyperoxia. The cells in the optic nerve head were found to exist in the same oxygen environment as cells of the inner retina. Oxygen tensions usually varied between 0 and 40 mmHg, with the mean being between 10 and 20 mmHg. There was no difference in oxygen levels between central and peripheral parts of the optic nerve head, nor was there a difference as a function of depth. Different levels of light adaptation of the retina had no effect on oxygen levels in the optic nerve head, but flicker stimulation of the retina resulted in decreases in tissue oxygen tension. A small number of experiments in which the intraocular pressure was raised showed that oxygen levels in the optic nerve head were not drastically altered during acute decreases in perfusion pressure. Based on the size of changes in tissue oxygen tensions during hyperoxia, it appeared that blood vessels in the optic nerve head constricted in response to systemic hyperoxia.