Metabolic faecal nitrogen excretion was assessed in 8 young rats (90-100 g body weight) following an oral application of 75 mg 15N-glycine (95 atom-% 15N). Four rats received an experimental diet containing 12% cellulose, while a control diet containing 4% cellulose was fed to the remaining 4 animals. The high cellulose content induced a highly significant reduction of the N balance due to a greatly increased urinary N excretion. The metabolic faecal nitrogen was determined by measuring 15N excretion in faeces and urine from day 5 (3) to day 8 following 15N glycine administration. During this time interval 15N elimination follows an exponential curve. Increasing the dietary cellulose content from 4 to 12% produced a rise in metabolic faecal nitrogen from 13.9 to 15.7 mg/day and in total faecal nitrogen from 21.3 to 24.4 mg/day. From these values a mean true protein digestibility of 98% was calculated for both groups of rats, regardless of the difference in dietary cellulose content. The fraction of endogenous faecal nitrogen which is of bacterial origin was determined through the analysis of 2, 6-diamino-pimelic acid (DAP). The added cellulose in the experimental diet caused a rise in faecal DAP from 0.302 to 0.402 mg/day corresponding to an increase of bacterial nitrogen from 5.2 to 6.4 mg/day. Accordingly the observed rise in endogenous faecal nitrogen is largely due to increased bacterial nitrogen.