Expts. on rats with diets of low protein-high carbohydrate, low protein-high fat, and high protein-low carbohydrate content and bearing the same amt. of naturally occurring Se indicate that its toxicity is detd. by dietary factors. Expressed as gms. protein % to [gamma] Se per 100 gms. of diet a protein: Se ratio of 1:100 is highly toxic, while a ratio of 1:30 or better is scarcely toxic in rats ingesting 10 p.p.m. Se in their diet. The same level of intake of Se in a low protein-high fat diet for a time causes stunted growth and alopecia but no tissue damage of a pronounced or permanent character. The bearing of these findings on the Se problem as related to livestock and man is discussed.