hallucinogenic alkaloid found in the seed coats of a plant (Peganum harmala) of the Mediterranean region and the Middle East, and also in a South American vine (Banisteriopsis caapi) from which natives of the Andes Mountains prepared a drug for religious and medicinal use. Chemically, harmine is an indole hallucinogen that can block the action of serotonin (the indole amine transmitter of nerve impulses) in brain tissue. Harmine occurs as the free alkaloid and may be converted to the hydrochloride salt, which is more soluble. Both are crystalline in form.