To state that oxygen brings about the retinopathy of prematurity seems an over-simplification. The objective of this paper is to present some of the physiological background upon which retrolental fibroplasia develops and to examine the possible etiologic relationship of factors other than oxygen therapy. In this examination certain thoughts may possibly be stimulated in regard to current thinking about other problems in vascular disease of the retina. The approach is not from statistical analysis of data but rather from the physiological point of view. There is little purpose in presenting here a history of the disease or the sequence of researches that have led to the general acceptance of high oxygen administration as its exciting cause. The immature infant is a complex being whose various organ systems have been presented, without adequate preparation, with an imposing work load. The term "immature infant" was chosen as best representing that group of