Isatin (1H-indole-2,3-dione) is a naturally occurring heterocycle that was first synthesised by Erdmann and Laurent in 1840. Isatin possesses a wide range of biological activities. Isatin has anxiogenic, sedative, anticonvulsant activities and acts as a potent antagonist on atrial natriuretic peptide receptors in vitro. Isatin is an endogenous monoamine oxidase (MAO) inhibitor involved in stress and anxiety. Additionally, isatin inhibits alkaline phosphatase (ALP), nitric oxide (NO)-stimulated soluble guanylate cyclase, and other enzymes.