Urea is the chief nitrogenous end product of the metabolic breakdown of proteins in all mammals and some fishes. It occurs not only in the urine of mammals but also in their blood, bile, milk, and perspiration. Urea, the diamide of carbonic acid. Urea has important uses as a fertilizer and feed supplement, as well as a starting material for the manufacture of plastics and drugs. It is a colourless, crystalline substance that melts at 132.7° C (271° F) and decomposes before boiling.