Tribulin is the name given to a family of endogenous nonpeptide substances which inhibit monoamine oxidase (MAO) and benzodiazepine binding. It is widely distributed in mammalian tissues and body fluids, and exhibit some species and tissue variations. Several components selectively inhibiting MAO A, MAO B, central and peripheral benzodiazepine binding (tribulins A, B, BZc and BZp, respectively) have been recognised. Tribulin A represents some tissue-specific metabolites of trace amines, whereas isatin is the major component of tribulin B. Tribulin content increases in brain under conditions of stress and anxiety and is reduced under sedation. Changes in tribulin content in the brain are accompanied by corresponding changes in the content of monoamines and their acidic metabolites, and also by altered susceptibility of MAO to specific mechanism-based inhibitors. This suggests that tribulin is involved in MAO inhibitory regulation in vivo.