In the gastro-intestinal tract the complex food proteins, under the hydrolytic action of enzymes, break down into a variety of substances, all of which belong to the group of a-amino-acids. These same absorbed from thence into the blood are transported to the various organs and tissues. Arrived thither the amino-acids are subjected to a process of sifting. Thus some are invoked for the reconstruction of broken down proteins, _i.e._, are re-synthesised into the body’s own characteristic tissues. The surplus amino-acids, viz., those not required for purposes of cell repair, undergo deaminisation. Two residues then result, one represented by ammonia, and the other by the remaining relics of the amino-acid molecule. The former is excreted as _urea_ and the latter is oxidised to produce energy. But there is yet another source of amino-acids, viz., the disintegration of tissue protein. To this end almost all bodily tissues possess intracellular enzymes capable of converting their proteins into the same simple products from which they took origin. Comparably with those of exogenous origin, these amino-acids of _endogenous_ formation undergo a like deaminisation; in other words, the bulk of their carbon, oxygen, and hydrogen is oxidised to form CO₂ and water, the residue combining with nitrogen to form _urea_, etc. The main end-product, then, of protein metabolism is _urea_, with traces of its forerunner _ammonia_. But there are also other waste nitrogenous metabolites. Thus, of the various amino-acids that become built up into tissue protein, some subsequently break down into products not belonging to the amino-acid category, viz., _creatine_ and _creatinine_. Some of the amino-acids, too, are excreted unchanged in the urine. Lastly, to these must be added those closely related substances, the _purin_ bodies, the end-products of _nuclein_ as opposed to general protein metabolism, of which latter _urea_ is the terminal product. To sum up, in a man on ordinary diet about 90 per cent. of his total nitrogen is excreted as urea, about 3 per cent. as ammonia, the residue of the nitrogen appearing in the form of other nitrogenous metabolites. SEAT OF FORMATION OF UREA The _liver_, it is generally held, is the main centre wherein urea is produced from the amino-acids; but not exclusively so, for it has been definitely established that, _even after removal of the entire liver in animals_, its production may not cease.