Three colostomised laying hybrids received 91.40 mg L-lysine-15N-excess (15N') each over a period of four days in a metabolism experiment with 15N-labelled lysine. After another four days, during which the hens received the same rations supplemented by commercial L-lysine, the animals were butchered and divided into individual fractions. After the hydrochloric hydrolysis of the organs and tissues the heavy nitrogen of lysine, histidine and arginine were separated, quantitatively evaluated, processed and measured with an emission spectrometer. Atom-% 15N' on an average amounted to 0.20 in the liver, 0.16 in the kidneys, 0.06 in the meat and 0.05 in the bones. Of the rediscovered 15N' applied, faeces contained 8.1%, urine 18.3%, the eggs 24.3%, the blood 4.9%, the meat 20.5%, the bones 5.2%, the gastro-intestinal tract with its contents 4.5%, the liver 3.5%, the kidneys 0.9%, the reproductive organs 3.7% and the rest 6.1%. The quota of rediscovery of the 15N' applied was 95.7%. 62% of the total 15N' was rediscovered in eggs, body and faeces as lysine 15N'. There was significantly more 15N' in the arginine in all fractions than in the histidine. The quota of the lysine-15N' of the total 15N' differed considerably in the fractions: less than 40% bones and blood; 48...56% gastro-intestinal tract, faeces, oviduct, kidneys; 62...63% remaining ovary, rest; 69...71% eggs, meat, liver. It could be proved that the alpha-amino group of lysine is to a large extent incorporated into other amino acids. Further proof that the amino acid metabolism proceeds in two phases was submitted, i.e. hight amounts of amino acids previously deposited in the body are used for egg synthesis.