β-Carbolines occur in man and rat. The concentration in various tissues is about 100 to 1000 times lower than that of classical neurotransmitters. Administration of β-carbolines in animals induces overlapping but not identical activity profiles. The molecular modes of action differ. For example, harman (1-methyl-β-carboline) acts as an endogenous inhibitor of monoamine oxidase [E.C. 1.4.3.4.], subtype A, whereas norharman (β-carboline) probably acts by stimulation of a specific β-carboline receptor which is different from the benzodiazepine-GABA receptor complex. There is substantial evidence that tetrahydroisoquinolines occur under physiological conditions as well. Whether tetrahydropapaveroline serves as a precursor of morphinanes in mammals, as has been found in opium poppies, remains to be elucidated.