The Sharanahua and Culina, small Indian tribes located in the southwestern Amazon basin, use a hallucinogenic drink for medicinal and social purposes. This decoction, called " Ayahuasca " in Peru, is prepared from Banisteriopsis Caapi stems and Psychotria sp. leaves. These plants have been botanically identified on the basis of voucher herbarium specimens and investigated for alkaloid content by means of a gas chromatography-mass spectrometry technique. A list of other occasional plant admixtures is given. Harmine, Harmaline, Tetrahydroharmine, Harmol and 6-Methoxytryptamine have been found in Banisteriopsis Caapi. Dimethyltryptamine, Monomethyltryptamine and 2-methyl-1, 2, 3, 4-tetrahydro-~-carboline have been found in Psychotria viridis and Psychotria carthaginensis. Harmine, Harmaline, Tetrahydroharmine and Dimethyltryptamine have been found in the drink. Quantitative calculations show the amount of each alkaloid administered in the Ayahuasca drink.