The effects of protein replacement by digestible fibre fractions "DgF" (hemicellu-loses + water-insoluble pectins) on the digestive physiology of the weaned rabbit were analysed by comparing four diets with a linear increase of the digestible fibre:crude protein ratio "DgFP" (1.01, 1.16, 1.34 and 1.55 for DgFP1 to DgFP4 groups, respectively), without changes of poorly digestible fibre levels (acid detergent fibre = 190 g/kg). 12 male rabbits were allotted to each of the four diets at weaning (32 days), housed in metabolism cages up to 71 days of age, and fed freely one of the four diets. Faecal and urine excretion were individually measured from 43 to 47 days of age, and from 63 and 67 days. At 71 days of age, the caecal contents of DgFP1 and DgFP4 groups were sampled to assess microbial activity (pH, volatile fatty acids concentration, etc.). The organic matter digestibility was similar for the four diets, but decreased by 2 units (0.65-0.63, P<0.001) with age (from 45 to 65 days) and without interaction with diet effect. As the intake of DgF increased by 20% from DgFP1 to DgFP4, the digestion of fibres increased (+9 and +15 units for NDF and hemicelluloses respectively, P<0.05). A diet by age interaction (P<0.05) was detected for crude protein (CP) digestion and retention: a 8% decrease in the digestibility of CP was observed between DgFP1 and DgFP4 for 7 weeks old rabbits, while for 9 weeks old rabbits this decrease reached 15%. While the faecal excretion of nitrogen remained unaffected by the diet, a 60% decrease in urinary nitrogen excretion was observed between DgFP1 and DgFP4 (P<0.001). The quantity of nitrogen excreted via the urine exceeded that of faeces for the DgFP1 group, but was 50% lower via urine than via the faecal route in the DgFP4 group. The nitrogen retention coefficient increased linearly (P<0.001) from DPF1 to DgFP4 (P<0.001), both in the post-weaning period (+5 units) or in the finishing period (+7 units). The ratio of retained nitrogen to digestible nitrogen increased linearly (P<0.001) between DgFP1 and DgFP4, and decreased substantially with age (-23 to -40% according to the diet). Although the caecal pH seemed to be lower (P=0.12) in the DgFP4 group, the total VFA and ammonia concentrations were similar between DgFP1 and DgFP4. High dietary DgFP ratios are thus recommended to reduce nitrogen output in urine; particularly at the end of the growing period when a reduced protein supply also decreases faecal nitrogen excretion. (C). 2013 Elsevier B.V. All rights reserved.