Six wethers, fitted with permanent catheters in the femoral artery and splanchnic blood vessels, were used in a balanced, incomplete block design to evaluate the effect of increasing the proportion of concentrate (cracked maize:soyabean meal, 90:10; T0, 0 g/kg DM; T30, 300 g/kg DM; T60, 600 g/kg DM) in the diet, at near maintenance energy intake (385±5.0 kJ EM/kg^0.75), on O₂ consumption and net flux of energy metabolites and NH₄⁺ across the portal-drained viscera (PDV) and liver. Increasing the proportion of concentrate in the diet altered PDV NH₄⁺ absorption (P<0.001; quadratic effect, P=0.04), liver NH₄⁺ removal (quadratic effect, P=0.08) and ß-OH-butyrate production (quadratic effect, P=0.09). However, the increased concentrate:forage ratio did not affect O₂ consumption by the PDV, or the net flux of glucose and lactate across the PDV, liver or splanchnic tissues. O₂ consumption by the liver tended to decrease (linear effect, P=0.06) with increased concentrate:forage ratio in the diet. Increased concentrate:forage ratio might increase nutrient availability to peripheral tissues by reducing the energy expenditure of the liver.