Raised arterial carbon dioxide levels have been suggested as a risk factor for retinopathy of prematurity (ROP). We investigated the effect of raised inspired CO 2 on normal postnatal vasculogenesis in the neonatal rat retina. One hundred fifty newborn rat pups were divided among 15 mothers (n = 10 for each litter). Five litters were exposed to low CO 2 (0.2%), 5 litters exposed to 6%, and 5 litters to 10% CO 2. On day 7 of life the rats were sacrificed and the total retinal and vascularized retinal areas analyzed. The vascularized retinal area and ratio of vascularized to total retinal area were reduced in rats exposed to 6% and 10% CO 2. Raised inspired CO 2 was associated with retardation of normal retinal vascular development and increased peripheral avascular area in neonatal rats. Raised CO 2 may be a risk factor for the development of abnormal neovascularization such as in ROP.