Retinal vessels of newborn and premature infants were photographed under conditions of increasing arterial oxygen tension and the changes in vessel diameter were measured. In 6 infants studied, there was a consistent pattern of vasoconstriction with rise in Po. The reduction in calibre could be measured, but was not detectable by viewing the retina, unless photographs taken at low and high Po were viewed side by side. Frank retinal vasospasm did not occur even at a Po of 250 mm. Hg. It was concluded that ophthalmoscopy did not provide an alternative to direct frequent measurement of Po in premature infants receiving oxygen therapy.