Metabolizable energy intake and heat production were measured in a series of calorimetry experiments carried out at the Agricultural Research Institute of Northern Ireland, Hillsborough, between 1993 and 1996 with beef cattle and sheep. A total of 75 estimates were made with cattle: 23 with Charolais cross steers; 16 with Simmental cross steers and 36 with Angus cross steers (450–628 kg liveweight). Fifty-six estimates were made with lambs: 24 with Blackface cross, eight with Suffolk cross and 24 with Texel cross (23–53 kg liveweight). The diets offered to both cattle and sheep contained proportionately 0·0–0·8 cereal-based concentrates, the remainder being grass silage. Linear regressions of energy retention (measured by calorimetry) against metabolizable energy intake were produced for the cattle and sheep studies. From these linear regressions an estimate of metabolizable energy required for maintenance (MEm) was obtained. For cattle, the derived MEm was 0·614 MJ/kg LW0·75 per day, and for sheep the derived MEm was 0·460 MJ/kg LW/kg LW0·75 per day. The estimates were proportionately 0·34 higher in cattle and 0·32 higher in sheep than the 1990 values of the UK Agricultural and Food Research Council.