In intensive animal production, surplusses of nitrogen (N) are polluting the environment. Therefore it is important to compose diets matching the requirements of the animals as close as possible. Untill now mostly apparently digestible N or amino acids (feacal) have been used as criteria. It is also well established that amino acids disappearing from the hindgut do not contribute much to amino acid supply of the animal. Moreover, big differences between ileal digestible amino acids and faecal digestible amino acids are found, depending on the feed composition. Because the amounts of absorbed amino acids is more reflected by the amounts of ileal digestible amino acids than by the amounts of faecal digestible amino acids, contents of the ileal digestible amino acids of the commonly used feed stuffs in the Netherlands have been determined. Diets can now be formulated on contents of ileal digestible amino acids.