I. Lipid peroxides have been measured by a micro-iodimetric method in pancreas, lung, adrenal, leg muscle, spleen, brain, small intestine and uterus of vitamin E-deficient and vitamin E-supplemented rats. There were no differences due to the vitamin E status of the rats. 2. The decay of [14C]~-a-tocopherol in peroxidizing methyl oleate, maize oil methyl esters and cod-liver oil methyl esters was studied at goo. When the decrease in tocopherol was plotted against increase in peroxide value for each ester preparation, it was found that, the greater the degree of unsaturation of the substrate, the less was the amount of tocopherol destroyed at any peroxide value attained before the end of the induction period. When the decay of tocopherol was plotted against time, however, the greater the degree of unsaturation of the substrate, the greater was the amount of tocopherol destroyed at any given time before the end of the induction period. 3. The significance of these results in connexion with the biological function of vitamin E is discussed. Bunyan, Murrell, Green & Diplock (1967) found that the concentration of lipid peroxides remained constant in several tissues of the rat regardless of its intake of unsaturated fat and vitamin E. We have now measured lipid peroxides in several other tissues of the rat and have studied the effect of vitamin E on them. Our findings, both previously and in this paper, seem incompatible with the idea that generalized lipid peroxidation occurs in tissues in the absence of vitamin E or that the role of a-tocopherol is that of a biological antioxidant. However, other workers (Witting, 1965a, b; Witting & Horwitt, 1964a, b; Witting, Harmon & Horwitt, 1965) have explained the effects of dietary unsaturated fat on vitamin E deficiency disease in terms of the ' peroxidizability ' of the dietary fatty acids incorporated into tissues and have attempted to treat the relationship between vitamin E and lipid in vivo and in vitro as mechanistically and kinetically identical. We have therefore investigated certain aspects of the relationship between fatty acid peroxidation and a-tocopherol in vitro and discuss in this paper the relevance of the findings to the problem of vitamin E function in vivo.