Vitamin A and its precursor β-carotene are naturally occurring antioxidants. The effects of diets deficient in β-carotene and vitamin A on the alveolar-capillary membrane were studied in young adult BALB/C mice before and after exposure to 65% oxygen. One of three diets (standard complete, β-carotene deficient, or β-carotene and vitamin A deficient) was fed for a 6-wk period. Mice were then exposed to 65% oxygen for 0, 3, or 6 days, sacrificed, and their lungs examined by electron microscopy using the morphometric techniques of Weibel. The arithmetic and the harmonic mean thicknesses of the alveolar-capillary membrane and its components (epithelium, interstitium, and endothelium) were measured to assess the influences of diet and of duration of exposure to 65% oxygen. Analysis of variance and multiple comparisons of means (Student-Neuman-Keuls statistics) were applied. Diet alone did not significantly affect membrane thickness. However, duration of oxygen exposure increased the thicknesses of both the epithelium and interstitium in the group fed a diet deficient in both β-carotene and vitamin A as compared to the other two groups although this was only significant for the epithelium. β-Carotene deficiency alone did not affect the respiratory membrane either before or after oxygen exposure. These results suggest that vitamin A may be an important nutrient in the protection against pulmonary oxygen toxicity.