Pigs of varying liveweight received protein-free diets in a trial made to establish the excretion levels of endogenic intestinal nitrogen and endogenic intestinal amino acids. The highest excretion rates of endogenic intestinal N and intestinal amino acids per kg of liveweight or per 100 g of dry matter ingested were found in pigs weighing about 10 to 40 kg. The lowest values were abtained in pigs with a liveweight exceeding 40 kg. The levels of endogenic intestinal N and intestinal amino acids are variable quantities. So, true amino acid digestibility data should be determined in pigs weighing more than 40 kg, because in these animals the relation between the intaken of amino acids and the level of endogenic excretion in the faeces is widest and hence, this variable factor would have the least influence on calculations of digestibility data. The data established for the true digestibility of nitrogen and amino acids of barley fed to pigs weighting 10-100 kg were found to be at the same level (with the exception of lysine) when due consideration was given to figures for the excretion of endogenic intestinal N and intestinal amino acids taking into account the liveweight of the animals.