Ingestion of heat polymerized linseed oil was followed by lowered growth rates in rats, this growth depression becom ing more severe with increased lengths of heating time of the oils and with increased amounts of oil in the diet (Crampton et al., '51). Our earlier experiments indicated that the low nutritive value of heated linseed oil diets is attributable to the oil itself and not to the effects of the heated oil upon non-lipid diet constituents. As thermal treatment causes oil to be less stable to oxida tion, it has been proposed that biologically deleterious prod ucts may accumulate through progressive autoxidation during storage of the oil-containing diets at room temperature. Oven temperatures used during the baking of the diets may also promote oxidative changes. The extent of autoxidation can be controlled by the addition of an anti-oxidant. A mixture of nordihydroguararetic acid (NDGA) with citric acid has been shown to be an effective anti-oxiclant for edible fats, increasing stability even in baked products (Mattil et al., '45; Higgins and Black, '45). The question of vitamin E deficiency in heated oils also seemed worthy of consideration at this time, since polymeriza tion at 275°C.would destroy any of this vitamin inherent in the original oil. The absence of vitamin E, a natural antioxidant, presumably permits a more rapid onset of autoxidation. In addition, a relation of vitamin E to growth, over and above its stabilization of linoleic acid, has been shown in insects by Fraenkel and Blewett ('46). In the feeding of heated lard to rats, Morris and his co-workers ('43) noted a depression of growth and also a paralysis simulating a vita min E deficiency. Therefore it was possible that in our tests thermal destruction of vitamin E was responsible in part for the adverse effects we had observed to result from feeding heat-polymerized linseed oil.