In rabbits, caecal fermentation relies to a large extent on the type of substrate available for bacteria. Therefore, in order to study the dietary effect of the source of carbohydrate on microbial nitrogen (N) absorption, thirty-two New Zealand growing male rabbits were randomly assigned to four diets formulated using two sources of structural carbohydrates (fibre), alfalfa hay (AH) and sugar beet pulp (SBP), combined with two sources of non-structural carbohydrates (starch), wheat or maize, at a constant fibre/grain sources ratio (0.80/0.20). Microbial N intake was estimated by preventing caecotrophy with a neck collar and, indirectly, by using urinary purine derivative (PD) excretion and microbial 15N-lysine incorporation. No effect of diet on growth was detected (average growth rate 26.6±0.69g/d), although dry matter (DM) intake was greater in animals fed diets with AH as main source of fibre than those receiving SBP (100.2 vs. 90.1g/d; P<0.01). Nonetheless, the latter diets were better digested and no significant differences were observed in digestible organic matter (OM) intake. Between sources of starch, digestibility of DM, OM and N was greater with wheat than maize (P<0.05). Microbial activity in the caecum was stimulated by SBP diets, as indicated by a greater volatile fatty acid concentration (89.6 vs. 67.5mmol/L; P<0.01) and a lower pH (5.7 vs. 6.2; P<0.001). Significantly higher amino acid 15N-enrichments in both caecotrophes and liver were observed with SBP diets and also in maize-fed rabbits when SBP was the main fibre source. However, microbial contribution to tissue amino acids (0.37±0.008) was not affected by the type of fibre.