The effect of vitamin E (tocopherol) on oxygen-induced retinopathy was studied in the kitten after a quantitative scoring system was developed for their India ink injected retinal flat preparations. In 75 previously described kittens exposed to two or three days of oxygen from day 3, treatment from day 1 with vitamin E or placebo disclosed that kittens treated with vitamin E had less retinopathy. Theories of the mechanism of action of vitamin E would predict that, if given only after the oxygen exposure, vitamin E should be ineffective. This was tested in 37 kittens with placebo or drug begun only after withdrawal from oxygen. Unexpectedly, significantly less intravitreal neovascularization was found in kittens treated with vitamin E after oxygen exposure.