PO2 measurements using a double barrelled recess type microelectrodes were measured in the optic nerve head of miniature pigs in normoxia and hyperoxia. Although periarteriolar PO2 increased markedly during systemic hyperoxia (100% oxygen breathing), the intervascular PO2 remained stable under hyperoxia. These results suggest a regulation of the tissue PO2 of the optic nerve head, as the retina does, at constant values during hyperoxia.