Four female sheep nourished wholly by infusions of volatile fatty acids, buffer and minerals into the rumen and casein into the abomasum were given, in addition, infusions of fermentable carbohydrates into the terminal ileum. The ileal infusions consisted of (1) water alone, (2) 25 g starch + 50 g cellulose, and (3) 50 g starch + 50 g cellulose. Measurement of nutrient digestibility and nitrogen retention were made over 5 days and the kinetics of urea metabolism were measured over 24 h by means of a single injection of [14C]urea. Endogenous urinary nitrogen excretion was measured over a subsequent 5 days when casein was omitted from the infusion mixtures. Increases in hindgut fermentation resulted in a significant increase in faecal nitrogen excretion (P less than 0.01) and a corresponding reduction in urinary urea nitrogen excretion (P less than 0.05). The ileal infusions did not significantly affect urea irreversible loss rate or urea pool size and were also without effect on plasma urea or rumen ammonia concentrations. Urea degradation in the gastrointestinal tract increased by about 2 g/day in progressing from lowest to highest level of hindgut infusion but differences between treatments were not statistically significant. Endogenous urinary nitrogen excretion was not affected by hindgut fermentation and averaged 206 mg N/(kg0.75 day) over the three treatment groups. Faecal nitrogen excretion however increased progressively with increase in ileal infusions (P less than 0.05) and was similar to that seen when nitrogen input was adequate. It is concluded that changes in hindgut fermentation can alter the partition of nitrogen excretion between faeces and urine but the quantities involved are small relative to the total exchange of urea across the digestive tract. The rumen appears to be an important site of urea degradation even when microbial fermentation is absent.