Naturally occurring benzodiazepines (BZDs) were first detected in mammalian tissues in 1986. They comprise a variety of 1,4-benzodiazepines corresponding to drugs commercially available for the treatment of anxiety disorders, sleep disturbances and epileptic seizures. Several biosynthetic pathways leading to the formation of BZDs are currently being discussed and have led to the proposition of possible precursor molecules. For years, the identification of naturally occurring BZDs in mammalian organisms was mostly confined to post mortem CNS material for sensitivity reasons. While radioimmunoassay and radioreceptor assay techniques have been tentatively applied to quantitations of genuine BZDs from human milk and cerebrospinal fluid, accurate measurements in peripheral blood have only recently become accessible, e. g., by gas chromatography/selected ion monitoring-mass spectrometry (GC/SIM-MS). This review summarizes existing evidence of benzodiazepines' occurrence in nature and discusses implications for neuropsychiatric disorders.