live weight (LW) of 2.03 (SE 0.185) kg, aged 70 d, each fitted with a wooden neck collar to prevent caecotrophagy. They were fed on five experimental diets formulated with different levels of nucleic acids (0.00, 3.75, 730, 11.25, 15.00 g yeast-RNA/kg diet). The relationship between intake of purine (x, pmol/kg W"'75) and total urinary PD excretion b, pmol/kg WO"'), y = 0.56 + 0.67~(r 20 .86; RSD 0.338), indicated that about 70 % of duodenal PB were recovered as urinary PD and that the endogenous contribution was constant and independent of dietary PB supply. Endogenous excretion of PD (allantoin and uric acid) was measured in a second experiment using six rabbits fed on a purine-free diet and fitted with neck collars to avoid caecotrophagy. Basal daily urinary excretion values for allantoin and uric acid were 532 (SE 33.9) and 55 (SE 7.3) pmol/kg W0'7' respectively; xanthine and hypoxanthine were not found in urine samples and therefore the sum of allantoin and uric acid should comprise the total excretion of PD (588 (SE 40.1) pmol/kg W0'75). The xanthine oxidase (EC 1.2.3.2) activity in plasma, liver, duodenum, jejunum and kidney was measured in a third experiment. The activities of xanthine oxidase in duodenal and jejunal mucosa, liver and kidney were: 0.61 (SE 0.095), 0.37 (SE 0.045), 0.035 (SE 0.001) and 0 units/g fresh tissue respectively and in plasma 2.96 (SE 0.094) units/l. The results show that urinary excretion of PD may be a useful tool to estimate duodenal PB input and microbial protein intake once the relationship between PB and N has been established in caecal micro-organisms.