I nvestigations using ammonium citrate-¹⁵N and urea-¹⁵N showed that children in the acute stage of kwashiorkor and marasmus receiving a diet of adequate protein content retained a considerable percentage of the label from both compounds. Excretion of both total ¹⁵N and urea-¹⁵N was subnormal and elimination was virtually completed 36 hr after administration of the isotope. During recovery from kwashiorkor total ¹⁵N excretion had approached normal a month after commencement of rehabilitation. Urea-¹⁵N excretion was still slightly subnormal after 3 months. In marasmus urea-¹⁵N formed a normal proportion of total ¹⁵N excretion after 1 month, although total ¹⁵N excretion then was still low. Ammonia nitrogen was retained to a greater extent than urea nitrogen in all cases. As it is known that a considerable amount of urea is degraded to ammonia in the gastrointestinal tract, it seems probable that urea nitrogen became available for use after this degradation. Examination of blood from one marasmic child after feeding ammonia-¹⁵N and from another after intravenous injection of urea-¹⁵N showed incorporation of the label into blood cells and plasma proteins. This did not occur in well nourished controls. It is concluded that ammonia and urea as sources of nonessential nitrogen may play an important part in protein metabolism in the malnourished child.