A six week dose-titration study was conducted to evaluate how selected dietary vitamin E levels affect by-products of in vivo oxidative reactions in companion animals. Forty dogs and forty cats (healthy adults) were randomly assigned to four equal groups per species. Control groups for both species were fed dry food containing a baseline level of Vitamin E. Treatment groups were fed the same food with vitamin E added at three different concentrations. Results show that normal dogs and cats experience oxidative damage and that increased dietary levels of antioxidants may decrease in vivo measures of oxidative damage.