Nitrogen balance studies were performed in normal adults and in children subsisting on typical New Guinea highland tuber diets. Only 3.5-5% of the total calories were derived from protein. Two adult groups, at a food-N intake of, respectively, 3.1 and 4.3 g N/d, and 3 child groups at intakes of 1.4, 1.6 and 2.3 g N/d, mainly from sweet potato, showed a marked negative N balance. One taro-eating child group, at an intake of 2.4 g N/d, was near equilibrium and one child group consuming sweet potato with skim-milk supplement, having 3.9 g N/d, showed retention. Faecal N, expressed as a fraction of food N, was always paradoxically high: 54-63% in the adult groups, 77-115% in the sweet-potato-eating child groups and 40% in the milk-supplemented group. In the adult groups, urine N was, respectively, 83 and 89% of food N. In the child groups, it was less: 53% in one taro-eating group and 40% in the milk-supplemented group. Despite a higher N intake in the taro-eaters, the utilization, as expressed in urine-N quantity, was less. Endogenous urine N (creatinine and uric acid) was about normal for body size. All sweet-potato-eaters excreted much hippuric acid. Urea N in the child groups was only 267-940 mg/d; in the adults, 1.8-3.2 g/d. The composition of faecal N was determined in one taro- and in one sweet-potato-eating group. A 22-24 kg boy, at intakes between 10-15 g protein (N × 6.25), produced daily 11-12 g crude protein in his stools, of which 8-11% was non-protein nitrogen. Most of the amino compounds in faeces emulsions were contained in fibrous, though digested, particles of over 500 µmdiameter, whereas 4-10% were present in the supernatant fluid. A comparison of the amino acids in taro and sweet potato and in the faeces of the consumers suggests that, on such diets, 60-80% of the amino acids in food are normally present again in the stools. This presupposes a particular autonomy of the intestinal flora in N metabolism, the significance of which for human nutrition is discussed.