A long-term N balance study was carried out to determine the adequacy of an estimated protein requirement level recommended for young healthy Nigerian women and the effect of the menstrual cycle on the requirement. Eleven healthy young women, 25 (SD 2.6) years, were fed on a diet providing 0.6 g protein (N x 6.25)/kg per d and an average energy intake of 0.17 (SD 0.012) MJ/kg per d. Urine, faeces, sweat and menstrual fluids were collected for estimation of N balance. Menstrual N loss varied among individuals ranging from 46 to 124 mg N/d with an average of 89 (SD 21.8) mg N/d. Individual N balance was found to vary according to the day of the menstrual cycle. Positive N balances were recorded at about ovulation while negative balances were observed just before the onset of menstruation. The average N balance ranged from +8.49 (SD 5.64) to -430 (SD 7.84) mg N/kg per d. Nevertheless, an overall cumulative positive N balance of +5.7 (SD 6.98) mg N/kg per d which did not change significantly with time was observed for the last 5 d of two consecutive 20 d diet periods, although three subjects were in negative N balance. Blood biochemical measurements were stable except for one subject who had elevated serum aspartate aminotransferase (EC 2.6.1.1) levels. These findings suggest that our estimate of protein requirements was sufficient to achieve N balance equilibrium in a majority (70%) of young women. However, to satisfy 97.5% of the population, slight adjustments might be necessary in the energy intake since subjects who were in cumulative negative N balance also lost weight.