Isatin is an endogenous indole with a distinctive distribution in brain and tissues. In the brain, the highest levels have been found in the hippocampus (0.1 μg/g), and an immunocytochemical stain has shown specific localization within particular cells. In vitro, its most potent known actions are as an inhibitor of monoamine oxidase B (ic50 ∼ 3 μM), and of atrial natriuretic peptide (ANP) receptor binding and ANP-induced guanylate cyclase (both with an ic50 ∼ 0.4 μM). In vivo, isatin administration (10–200 mg/kg) causes an increase of monoamine neurotransmitter levels in the brain. Isatin is anxiogenic in animal models at doses of 10–20 mg/kg and sedative at higher doses. Its anxiogenic effects are unlikely to be due to inhibition of monoamine oxidase, but may possibly stem from interaction with the ANP system. Isatin may mediate a link between monoamines and the natriuretic peptide system, and its analogues may provide new pharmacological tools.