 Dear students, in this topic, we shall discuss the nitrogenous wastes in animals. The catabolism of nitrogen-containing macromolecules results in the formation of toxic waste by products. The major nitrogenous waste produced in animal cells is the amino group. The 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 ammonia, as urea, or as uric acid. In addition to the amino group, many other nitrogenous waste-metabolic byproducts are also produced, but they are produced in less quantities. Such nitrogenous wastes include creatinine, creatine, and tri-methylamine oxide. Moreover, the metabolism of purene and pyrimidine bases of nucleic acids produce significant amounts of nitrogenous wastes, including the uric acid, hypoxanthine, xanthine, and allantoin. However, the byproducts of amino acid metabolism, that is, ammonia, urea, and uric acid, are considered as the major nitrogenous wastes. They are excreted by animals in large quantities. The other nitrogenous wastes are also excreted, but they are excreted in urine in very small amounts. The ammonia 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 one gram of ammonia requires about half a liter of water. To excrete one gram of ammonia of such a large amount, only aquatic animals can be afforded. And in those aquatic animals, ammonia is a major excretory product. Testial animals convert ammonia into urea or uric acid, which can be excreted with less water loss. Urea is less toxic as compared to ammonia, and it requires only 0.05 liter of water for the excretion of one gram of urea. That is, only 10% as compared to that of ammonia. 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 for one gram of excretion of uric acid. It is only 1% water loss as compared to that required for ammonia. Uric acid uses less water because it is slightly soluble in water. It is excreted as a white pasty precipitate. It can be excreted in a semi-solid form. That is why it does not require water loss. Dear students, during evolution, different animal groups have adapted to produce mainly one major excretive product, i.e. some animals excrete ammonia as a major nitrogen waste, some use urea and some use uric acid. In general, we can say that water availability and habitat of the animal determine the nature and pattern of nitrogen waste excretion.