The metabolic fecal nitrogen was determined in 8 young rats (90-100 g body weight) using a single oral dose of 75 mg 15N-glycine. Four rats were fed a diet containing 8% guar and 4 rats received a control diet without guar. Compared to the control group, apparent protein digestibility and nitrogen balance were significantly lower in the guar group. After an initial steep rise the 15N-elimination in feces and urine (days 2-8 after 15N-glycine application) followed an exponential curve in at least 2 phases, with increasing half-time values. The proportion of metabolic fecal nitrogen to total fecal nitrogen was calculated from the 15N-atom-% excess, determined daily in urine and in feces. This calculation is based on the assumption that the specific 15N-activity (15N-atom-% excess) in urine represents the specific 15N-activity of the endogenous nitrogen pool. The addition of guar raised the proportion of metabolic fecal nitrogen from 51% (control) to 73%, corresponding to an increase in total fecal nitrogen loss (24 mg) observed in the guar-fed animals. This indicates that the rise in fecal nitrogen loss caused by guar is due almost entirely to enhanced secretion of endogenous nitrogen and not to undigested food protein. Based on the analysis of 2,6-diamino-pimelic acid in the feces of 4 rats the proportion of bacterial nitrogen in the feces was calculated. It amounted to 19.8% of total fecal N in the control group, and to 30.1% in the guar group, or to 39.4% and 44.4% of metabolic fecal nitrogen respectively. Obviously the increased dietary fiber intake also caused a rise in the amount of metabolic fecal nitrogen, which is derived from gastro-intestinal secretions and epithelial tissue.