When least-cost diets, based on metabolizable energy (ME) values are formulated one assumes that the ME values allocated are additive, so that the sum of the individual ME values will represent the ME value of the diet. An experiment was undertaken to determine the true metabolizable energy (TME) values of a number of food components so that a complete diet can be formulated to test the additivity of the ingredients. Corrections for nitrogen, with and without endogenic nitrogen were made. The best additivity was found where the TME values were corrected for N-retention, taking endogenic nitrogen into account. The percentage difference from additivity was 4,8% for the TME values where no corrections was incorporated and 2,8% where nitrogen and endogenic nitrogen was taken into account.