Currently research on feed supplementation with natural compounds to improve energy and protein utilisation and to mitigate the greenhouse gas methane in ruminants is intensively pursued. Two compounds, diallyl disulphide (DADS), an important component of garlic oil, and lovastatin, an inhibitor of a key enzyme of methanogenic Archaea, were selected on the basis of their in vitro anti-methanogenic potential. In three 23-day experimental runs, six sheep received hay and concentrate in a duplicate 3 x 3 Latin square design. The concentrate was either not supplemented or supplemented with either 4 g DADS or 80 mg lovastatin per kg of total dietary dry matter. There were no refusals of concentrate for any treatment. Respiratory measurements were conducted on experimental days 7/8 (Period 1) and days 17/18 (Period 2). Relative to the control, digestibility of neutral detergent fibre (NDF) tended to increase (p = 0.09) with DADS by 14%. This was associated with an increased (p = 0.07) body energy retention of the animals. Effects on nitrogen balance and ruminal fermentation traits were never significant. No influence of supplements or period was found on total daily CH4 production which averaged at 28.6 g per sheep. However, across both periods the amount of CH4 produced per kg NDF digested was lower (-8%; p = 0.02) with DADS than without supplementation, and the lovastatin treatment ranged in between. In conclusion, the study demonstrated a certain potential of DADS to improve fibre digestion and body energy retention and to limit CH4 formation in relation to digestible fibre intake, while lovastatin remained ineffective.