Obligatory N losses through feces (FN) and urine (UN) were measured in five children and N balance was measured in them and in five others (23 +/- 4 months old) using four levels of cow's milk intake. FN, UN and FN +UN were 19.5 +/- 6.9, 34.0 +/- 5.3, and 53.7 +/- 8.1 mg N/kg/day, respectively. The ratios of FN, UN and total obligatory losses (FN + UN + sweat and integumental N) to basal energy expenditure were 0.38, 0.64 and 1.11 mg N/basal kcal, respectively. The two latter values are 32 and 45% lower than the 1971 FAO/WHO estimates. Mean N requirement determined by factorial calculations using a correction factor of 1.3 and by N balance techniques was 98 mg N or 0.61 g milk protein/kg/day, which is 33% lower than the FAO/WHO estimates. Depending on the allowances made for inter-individual variability, safe levels of protein intake ranged from 0.79 to 0.94 g/kg/day, 33 to 21% lower than FAO/WHO recommendations. Apparent and "true" milk protein digestibilities were 80 +/- 4% and 94 +/- 4%, respectively.