In vertebrates, deamination takes place particularly in liver cells, where the excess of amino acid supplied by eaten food is deaminated. Ammonia produced in this way is highly toxic and must be disposed of. Elimination oft en occurs in the form of uncharged ammonia or a more complex nitrogenous substance, which is either uric acid or urea. Based on the nature of the nitrogenous excretory end product, animals are classified into three main categories:

1. Ammonotelic
2. Uricotelic
3. Ureotelic