Caecal contents of 3-month-old rabbits were analysed and used as inoculi of cultures supplied with plant proteins (gluten, gliadin, zein), mucin, casein, ribonucleic acid (RNA) and glucose. The caecal digesta contained dry matter, total N, protein, trichloroacetic acid (TCA)-soluble N and NH3-N at 234, 10·2, 42·6, 3·2 and 0·27 mg/g (on average), respectively. One g of the caecal digesta contained 180 μmol of glycine equivalents of amino acids in TCA-soluble fraction. Because ninhydrin reaction was almost three times greater after HCl hydrolysis, peptides rather than amino acids were the primary component in this fraction. Urea concentration was only 2·3 μg N/g.

Net production of volatile fatty acids (VFA) in cultures with casein and mucin was higher than in cultures with gluten, gliadin, zein and RNA. Zein was the least fermentable substrate tested. Acetate was the principal fermentation end-product, followed mostly by butyrate and propionate. Branched-chain VFA proportions in VFA produced in cultures with casein, gluten, gliadin and zein were higher than in those supplied with glucose, mucin and RNA. In the former cultures, branched-chain VFA accounted for 10·2–17·7 molar % in the total VFA. In cultures supplied with proteins, there was a significant correlation between methane and VFA production, and between VFA and ammonia. The highest concentration of ammonia was found in cultures with casein. Production of methane was increased on glucose and nitrogenous substrates containing a carbohydrate moiety (mucin, RNA). It can be concluded that the caecal contents of rabbits are rich in nitrogenous compounds. The caecal fermentation of nitrogenous substrates supplies nutrients both for caecal microorganisms (branched-chain VFA, NH3) and for the host (VFA).