The effect of amino acid deficiency on the change of body composition in adult rats has been investigated. Six experimentaldiets, lysine-, methionine-, threonine-, tryptophan-, and protein-free, as well as the lactalbumin diet were prepared. The rats fed the lysine-free diet lost the least body weight while the rats fed threonine-, or protein-free diets lost body weight very rapidly. Themean weight loss in the methionine-, or tryptophan-free rats was between that of the lysine-, and threonine-free rats. The water contents in rats adequately reflected the weight loss. The rats that lost the least body weight showed the lowest water contents in their bodies and vice versa. Total body nitrogen contents in rats fed diets devoid of lysine, methionine, or tryptophan were not significantly different from the group fed the lactalbumin diet, whereas total body nitrogen contents in the group fed a threonine-, or protein-free diet were the lowest. Deficiency of lysine resulted in an accumulation of larger amounts of fat in the body as well as in the liver of the rats.