During the recovery period after a high oxygen injury in the kitten, chronic hypoxia adversely affects the resulting retinopathy, but increasing oxygen breathing to 28% improves it. To test the effects of chronic hypoxia without an antecedent hyperoxic injury in the kitten, the animals were raised in 13% or 21% (room air) oxygen and their retinas examined at 3, 7, 14, or 21 days. They were also studied after 14 days of exposure to 30% or 40% oxygen to compare the graded effects of elevated oxygen to that of hypoxia on the development of the retinal capillary bed. Chronic hypoxia alone did not affect somatic growth or cause retinopathy. An inverse relationship was found between the rate of retinal vascularization and ambient oxygen. As inspired oxygen rose from 13%-40%, the proportion of the retina vascularized at 14 days fell from 76 +/- 12%-18 +/- 8% (mean +/- standard deviation, P less than 0.01). All 14-day animals had similar capillary density at the advancing edge of their retinal vasculature (mean diameter of the capillary meshwork = 71 +/- 12 microns) despite the impairment of forward progress observed in elevated oxygen. However, the width of the periarteriolar capillary-free zone increased from 65 +/- 10 microns in 13% oxygen-breathing animals to 104 +/- 5 microns in 40% oxygen-breathing animals (P less than 0.001). The animals raised in hypoxic conditions had more mature-appearing retinal vasculature at 21 days than did controls.(ABSTRACT TRUNCATED AT 250 WORDS)