Total of seven teenagers and two adults were given sweet potato diets that supplied slightly below requirement levels of protein for 32 (experiment 1) or 53 days (experiment 2) in two separate experiments. In experiment 1 and during the first 36 days of experiment 2, nitrogen (N) balance studies with the teenagers were conducted with 0.67 and 0.71 g of protein per kilogram of body weight, respectively. Results of the N balances including skin N loss were -0.5 mg N/kg per day in experiment 1 and -3.2 mg N/kg per day in experiment 2. Two adults were given 0.63 g/kg of protein and gave average N balance of +6.0 mg N/kg per day (experiment 2), however, one of them had a slightly negative cumulative N balance if miscellaneous N losses were included in calculation. Plasma urea N of both teenagers and adults decreased significantly from 8 to 11 to 2 to 3 mg/100 ml in experiment 2. After 32 or 53 days on the sweet potato diets, the plasma free amino acid pattern of the teenager subjects showed abnormality. Furthermore, the subjects were easier to get fatigue by physical exercise, and took longer nap due to sleepiness during the latter days of the experiments. Thus, although the mean hemoglobin, hematocrit, plasma total protein, and plasma albumin levels were within the normal ranges, the protein nutrition status of the teenagers and one of the two adults was considered to be not normal. The results suggested that intestinal N-fixation that was proposed by several investigators to occur in sweet potato eaters probably did not occur. The subjects' feces were examined in vitro for the ability of N-fixation with negative results. During the last 17 days of experiment 2 the effect of excess calorie on N balance was examined.