 Dear students, in this topic we shall discuss the Nitrogenous Wastes in Animals. The Catabolism of Nitrogen-containing Metromolecules results in the formation of toxic waste by products. The major Nitrogenous Waste produced in animal cells is the Amino Group. Amino Group is produced by the Catabolism or the Amination of Amino Acids. Amino Group is highly toxic. It is removed by animals in three forms, as Amonia, as Urea or as Uric Acid. In addition to Amino Group, many other Nitrogenous Waste Metabolic by-products are also produced, but they are produced in comparatively less quantities. Such Nitrogenous Wastes include Cratinine, Creatine and Trimethyl Amine Oxide. Moreover, the Metabolism of Pyrene and Pyrimidine Bases of Nucleic Acids produce significant amounts of Nitrogenous Wastes including the Uric Acid, Hypoxanthine, Xanthine and Allantoin. However, the by-products of Amino Acid Metabolism i.e. Amonia, Urea and Uric Acid are considered as the major Nitrogenous Wastes. They are excreted by animals in large quantities. The Amonia is highly toxic. It needs to be immediately excreted out of the body. It requires a large amount of water for its excretion. That is, excretion of 1 gram of Amonia requires about half a liter of water. Such a large amount of water can only be afforded by aquatic animals to excrete 1 gram of Amonia. And in those aquatic animals, Amonia is a major excretory product. Crestial animals convert Amonia into Urea or Uric Acid which can be excreted with less water loss. Urea is less toxic as compared to Amonia and it requires only 0.05 liter of water for the excretion of 1 gram of Urea. That is only 10% as compared to that of Amonia. However, Urea synthesis is energetically costly as it consumes 80 pence. The excretion of Uric Acid requires least water. That is only 0.001 liter per 1 gram of excretion of Uric Acid. It is only 1% water loss as compared to that required for Amonia. Uric Acid uses less water because it is slightly soluble in water. This is not water soluble at all. It dissolves slightly in water and it is excreted as a white pasty precipitate. It can be excreted in a semi-solid form. That is why it does not have water loss. During evolution, different animal groups have adapted to produce mainly one major excretive product. Some animals excrete Amonia as a major nitrogen waste, some use Urea and some use Uric Acid. In general, we can say that water availability or habitat of the animal determines the nature and pattern of nitrogenous waste excretion.