Autoregulation of retinal blood flow was studied in seven normal volunteers. The stimuli to autoregulation used were hyperoxia and hypoxia under controlled conditions of isocapnia. Macular blood flow was measured using the blue field entoptic phenomenon, and retinal artery and vein diameters were measured using a computerized digitising system. Results under conditions of isocapnic hypoxia showed a 38% average increase in blood flow and an increase in diameter of 8.2% and 7.4% in retinal arteries and veins, respectively. In hyperoxia, blood flow fell by an average of 36% and retinal artery and vein diameter decreased by 5.6% and 10%, respectively.