1. Methane production rates were estimated simultaneously in the rumen and caecum of sheep given 200, 400, 600, 800 and 1000 g lucerne (Medicago sativa) chaff/d using isotope dilution techniques. Estimates were also made of volatile fatty acid (VFA) production in the rumen at each level of feeding. In all studies three to four animals were used at each level of intake. 2. Production of VFA and of methane were both related to digestible energy (DE) intake. Regression lines for both VFA production and methane production v. DE intake had significant intercepts indicating an input of endogenous, fermentable organic matter into the rumen in excess of 50 g/d. 3. The values obtained for rates of methane production were compared with those calculated from stoicheiometric equations relating rates of methane and VFA production. Comparisons of methane production with that predicted from DE intake were also made. 4. Balances for digestion of food determined for the rumen indicated that the energies in the end-products were more than 100% of the DE intakes of lucerne chaff. Correction for fermentation of apparent endogenous materials resulted in more realistic values. Endogenous materials appeared to make a significant contribution to VFA and methane production, particularly at low levels of intake.