Oxygen tension was recorded with double-barreled microelectrodes in the retina of anesthetized cats and monkeys. The oxygen tension (PO2) was often observed to undergo spontaneous irregular fluctuations with peak-to-peak amplitudes of about 3 to as much as 20 mm Hg. These fluctuations were observed only in the vascularized inner 40 to 50% of the retina, not in the avascular outer retina, and they were not correlated with the voltage recorded by the second barrel of the microelectrode. Oxygen records from the cat retina were subjected to Fourier analysis to determine their frequency content. The peak frequency was usually in the range of 3.5 to 10.5 c/min (0.06 to 0.18 Hz), considerably below the respiratory frequency of 20 to 30 c/min or the heart rate of 180 to 240 c/min. The power of the oxygen fluctuations fell gradually to less than 10% of the peak power by 60 c/min. It is suggested, based on the depth distribution and on comparison of these data with the literature on vasomotion, that the PO2 fluctuations are the result of changes in oxygen supply caused by normal fluctuations of arteriolar diameter (vasomotion) and/or capillary perfusion.