The effect of post-ruminal fermentation on faecal output of purine bases (PB) and urinary excretion of their derivatives (PD) was studied by the infusion of yeast RNA (2·9 g/day), corn starch (100 g/day), cellulose (200 g/day) and saline solutions into the gastro-intestinal tract offour sheep (35·4 kg live weight) fitted with caecal and duodenal catheters and given 0-80 kg/day alfalfa hay. All substrates were infused into the caecum except cellulose that was infused into the proximal duodenum.

The infusion of RNA did not affect either the faecal excretion of PB or the urinary excretion of allantoin or total PD, although xanthine excretion increased significantly from <0·01 to 0·12 mmol/day.

Starch and cellulose infusions promoted a significant increase in the faecal excretion of diaminopimelic acid (206 and 159 v. 131 mg/day) and PB (4·55 and 3·62 v. 2·29 mmol/day) and modified the partitioning of total nitrogen losses between faeces and urine. The urinary excretion of allantoin and total PD were not affected by the caecal infusion of starch (6·60 and 22·9 v. 7·02 and 22·8 mmol/day) but both tended to increase with the duodenal infusion of cellulose (7·75 and 26·4 mmol/day).

It is concluded that the urinary excretion ofPD is independent of either the supply of nucleic acids to the caecum or the extent of hind gut fermentation although it may be affected by variations in the flow of undigested fibre along the small intestine.