- It is the type of excretion in which ammonia is the main nitrogenous waste material. Such animals are called ammonotetic.
- It is found in aquatic animal groups like sponges, coelentrates, crustaceans, echinoderms, bony fish, tadpole larvae and salamander.
- Ammonia is produced as a result of catabolism of proteins, especially in the liver cells by oxidative deamination of excess of amino acids in the presence of oxidase enzyme.
- Ammonia is highly toxic and must be metabolised or expelled from the body as soon as possible.
- Ammonia is highly soluble in water and a very large volume of water is needed by the animal to dissolve ammonia. 1 gm of ammonia needs about 300 - 500 ml of water. But this is not a problem for animals living in an aqueous habitat which are generally found to be ammonotelic.

- It is a type of excretion where urea is the main nitrogenous waste material. Animals showing ureotelism are called ureotelic animals.
- Generally found in land animals which can afford to excrete sufficient volume of water or to concentrate urea in considerable quantity in the urine. It is commonly found in man, whales, seals, desert mammals like kangaroo rats, camels, toads, frogs, cartilagenous fishes, aquatic and semi aquatic reptiles like alligator, terrapins and turtles.
- In the liver of the animals, ammonia is detoxified to form urea by the orrithine cycle. Urea is far less toxic than ammonia and so can remain inside the body for a longer period without causing any ill effects inside the body.
# 1 gm of urea needs about 50 ml of water to the expelled out.

- Elimination of uric acid as the main nitrogenous waste material is called uricotelism. Animals showing uricotelism are called uricotelic animals.
- It is a common method seen in birds, land reptiles, insects, land snails and some land crustaceans.
- Uric acid is formed from ammonia mostly in the liver and to some extent in the kidneys. The process is highly energy dependant, but is much less toxic than both ammonia and urea and it is almost insoluble in water and can be eliminated from the body in nearly a solid state, saving a lot of water. Since kidneys can handle the nitrogenous wastes only in solution, reptiles and birds pass a dilute solution of uric acid into the cloaca, where water is absorbed and solid uric acid is eliminated along with faeces. The faecal matter of certain birds like cormorants, pelicans and gannets called guano has been used for the commercial extraction of uric acid. Islands off the coast of South America are covered with guano.
- Man also excretes a small amount of uric acid in his urine formed by the catabolism of nucleic acids.