Eleven health elderly women, ranging in age from 67 to 91 years, were given an essentially protein-free diet for 8 to 10 days, to determine obligatory urinary and fecal nitrogen losses. Body cell mass (BCM) was calculated from whole body 40K, and basal metabolic rate was determined by indirect calorimetry. Urine was analyzed daily for nitrogen and creatinine, and fecal N was measured in pooled samples. Urinary N output reached a relatively stable-state level by day 6; the average of the last four daily measurements was taken as an estimate of obligatory urinary N loss. The obligatory urinary N was: 24.4 +/- 5.2 mg N/kg body weight; 89.5 +/- 17.1 mg N/kg BCM; 1.44 +/- 0.14 mg N/basal kcal; and 2.11 g N/g creatinine. Obligatory fecal N was 9.8 mg N/kg body weight. Comparison of these results with published data for young adults indicates that obligatory urinary N losses in elderly women are similar to those for young women when expressed per unit body weight but higher than young women and men when expressed per unit creatinine excretion and per unit BCM.