Male, Sprague-Dawley (N.I.H.) rats with an average starting weight of 120 g. were placed in individual metabolism cages and allowed to imbibe ad libitum one of three water-soluble, chemically defined diets containing pure amino acids, with groups of five rats per diet. The dietary intake and urinary output were carefully measured volumetrically, and the total nitrogen of these fluids and the distribution of the urinary nitrogen among urea, ammonia, and amino acids were determined. Fecal nitrogen was also determined, the total excreted nitrogen subtracted from the total intake nitrogen yielding the value of nitrogen retention or nitrogen balance. The ratio of this value to the average daily weight gain (i.e., milligrams of imbibed nitrogen retained per day per gram of weight gain per day) appeared to be remarkably constant at 34–35 mg./g. for animals of all three soluble diets, even though each diet produced different rates of growth, and even though, in another experiment, very much larger animals were employed with a relatively low growth rate. The proportion of urea N in combined urea N and ammonia N levels was 95% for the best diet with a wide variety of “non-essential” amino acids (No. 26), and lesser for the poorer diets (Nos. 3 and 40). α-Amino acid nitrogen excretion in the urine was minimal (about 2–3% of the total nitrogen) on diets Nos. 3 and 26 which contain only l-amino acids, and considerable (about 25% of the total nitrogen) on diet No. 40 which contains several racemic amino acids. Five male, Sprague-Dawley (N.I.H.) rats on diet No. 26, and with an average starting weight of about 250 g., were placed in individual metabolism cages and maintained several more days on this diet. A soluble, amino acid-free diet (No. 49) was then substituted for diet No. 26, 20 days being required on the depletion diet to produce a weight loss of about 20%. The animals were then returned for 8 days on diet No. 26 to make up this weight loss. During this repletion period the average weight gain per day was about 6.3 g. per animal, and the ratio of nitrogen retained to weight gained turned out to be 18.2 mg./g.; this low value suggested a higher level of fluids or fat or both in the newly and rapidly regenerated tissues. The ammonia N excretion depends on the amino acid composition of the diet, and is minimal on diet No. 26. This minimal figure remains strikingly constant throughout the predepletion, depletion, and repletion stages, and probably reflects some homeostatic mechanism. The fecal N excretion is very small and constant, regardless of the nature of the soluble diet, or whether the animals were in predepletion, depletion, or repletion stages, which indicates an essentially quantitative absorption of the nitrogenous components of the soluble diets. The relation between intake of nitrogen and its retention is discussed.