SummaryTo find equations able to estimate the fermentation characteristics of the caecum from that of faeces, caecal content and faeces of 10 hybrid Hyla rabbits were used as inocula for an in vitro gas production trial. About 1 g of 12 roughages, 11 hays (ryegrass, alfalfa, sulla, oat, vetch, sulla‐lolium, vetch‐oat, sulla‐oat, clover, ryegrass‐clover, sulla‐vetch‐oat) and a wheat straw, was weighed, in triplicate per inoculum, in 120‐ml flasks; 75 ml of anaerobic medium and 4 ml of reducing solution were added and the flasks were placed at 39 °C. Caecal content and faeces were diluted respectively 1:2 (CI) and 1:8 (FI) with anaerobic medium and were introduced into their respective flasks (10 ml). Gas production was recorded 20 times at 2–24 h intervals throughout fermentation (120 h). The fermentation characteristics (i.e. degraded organic matter, OMd; potential gas production, A; maximum fermentation rate, Rmax; volatile fatty acid, VFA; ammonia, NH3) were studied by inocula and substrates. The two inocula did not differ in OMd but CI produced significantly higher gas (A, 213.1 vs. 199.4 ml/g, respectively, for CI and FI, p < 0.01) in less time (Rmax, 3.08 vs. 2.24 ml/h, respectively, for CI and FI, p < 0.01). CI also produced higher levels of total VFA (57.86 vs. 46.70 mmol/g OM, respectively, for CI and FI, p < 0.01) and showed a higher branched chain proportion (0.023 vs. 0.018, respectively, for CI and FI, p < 0.01). For some parameters (as OMd pH and propionate) the equations for the estimation of caecal fermentation characteristics from that of faeces were accurate (R2 > 0.8828) and reliable (CV < 10.78%) suggesting that faeces can be successfully used for the estimation of these parameters.