The presence of the fermentable sugar d-mannitol in the diet improves nitrogen (N) utilization in rabbits. To clarify the mechanism by which d-mannitol improves N utilization, we studied the effect of d-mannitol on the fate of blood urea N in growing rabbits. Growing rabbits received a control diet or a diet containing d-mannitol, which were formulated by adding 80 g/kg glucose or d-mannitol to timothy hay. After 9 days of feeding of the experimental diets, 15N-urea was administrated intravenously under anesthesia 1 h before slaughter. The blood urea level (concentration of both urea N (43.6% of the control group (CG), P < 0.05) and 15N (95% of the CG, P < 0.05) in blood serum) was reduced in the mannitol group. The concentration and amount of N, and 15N atom % excess in the contents of the cecum and colon were higher (P < 0.05) in the rabbits fed the mannitol diet than in rabbits fed the control diet, especially in the cecum. The consumption of mannitol caused bacterial proliferation in the cecum characterized by marked short-chain fatty acid production (165% of the CG, P < 0.05), decreased cecal ammonia N (73% of the CG, P < 0.05) and elevated cecal bacterial N (150% of the CG, P < 0.05). On the other hand, addition of d-mannitol to the diet decreased N (80% of the CG, P < 0.05) and 15N (77% of the CG, P < 0.05) excretion in the urine. These results indicate that d-mannitol increases the transfer of blood urea N to the large intestine, where it is used for bacterial N synthesis.