Previous papers have dealt with gain in weight1 and energy metabolism2 of undernourished children. The current article opens the discussion of the substance metabolism of the same children, and will deal with some of the urinary nitrogenous constituents. Nitrogen balance, mineral metabolism and caloric intake and output will be discussed in papers to follow. Very little material is available on the substance metabolism of undernourished children, since most investigators of the problem of malnutrition have studied infants. Fleming3 reports no difference in behavior of normal and marantic infants with respect to total urinary, creatinine and ammonia nitrogen, and Utheim4 finds normal creatinine values, but a high excretion of ammonia nitrogen concurrent with high organic acids which decrease with improvement in the nutritional state of the infant. EXPERIMENTAL Most of the subjects used in this work have already been reported for gain in weight and for basal