An energy system is described in which, in both single-stomachedand ruminant animals, the heat increment of feeding is considered to be linearly related to five measurable quantities. For both kinds of animals there of the quantities, with their heat increments in parentheses, are urinary N(wu;kJ/g),faec alorganic matter (wd; kJ/g) and positive protein retention (wp; kJ/g). Inruminants the other two, with their heat increments in parentheses, are CH4energy (wm; kJ/kJ) and positive lipid retention (w1;kJ/g); in single-stomached animals they are positive lipid retention from feed lipid (wu; kJ/g), and positive lipid retention not from feed lipid (w1; kJ/g). Data from suitable experiments on steers, pig sandchickens were used to test the system and to estimatewu29·2, wd3·80, wp36.5, wm0·616, w116·4 and w114·4. The values for wu, wd, wm and (wI–wII) allow an energy scale, called effective energy, to be defined for both single-stomached animals and ruminants. On this energy scale the values of wp and w1, to gether with the heats of combustion of protein and lipid of 23·8 and 39·6 kJ/g respectively, allow the energyr equirement to be expressed as (MH+ 50 PR+56 LR) for both kinds of animal, where PR and LR are the rates of positive protein an lipid retention (g/d), and MH is the maintenance heat production (kJ/d) which can be estimated as 0·96 of the fast in gheat production. The effective energy (EE) yielded toaruminant animal by a feeding redient can be estimated as EE (MJ/kg organic matter)=1·15 ME–3·84–4·67 DCP, where ME is the metabolizable energy value (MJ/kg organic matter)and DCP is the digested crude protein content (kg/kg organic matter) with both measured at maintenance. Alternatively, EE can be estimated as EE (MJ/kg)=GE (d–0·228)–4·67 DCP, where GE is the gross energy (MJ/kg)and d is the energy digestibility (MJ/MJ) also measured at maintenance. The EE yielded to a single-stomached animal can be estimated as EE(kJ/g)=1·17 ME–4·2 CP–2·44, where ME(kJ/g)is measured at, orcorrected to, zero N-retention and CP (g/g)is the crude protein (N×6·25) content of the feeding redient. The system is simpler for ruminants, and more accurate for both kind soft animal, than those no win use. As effective energy values can be tabulated foring redients, and are additivet othe extent that ME values are additive, they can be used to formul at ediet susing line ar programming.