Hallucinogenic plants are used in the primitive societies for centuries, but so far they were only used by some shamans, healers, etc. during religious rites. Nowadays a renewal of interest is noticed for natural hallucinogens in connection with a return-to-nature movement because the spreading of these substances has been recently more easier and increased through internet. Among these hallucinogens, Ayahuasca is the most wellknown. Ayahuasca is a mixture of several plants from the northwest of South America. For its preparation, two sorts of Banisteriopsis, a giant liana, are particularly important and sources of (beta-carboline alkaloids : harmaline, harmine and tétrahydroharmine, harmol which are natural hallucinogens). Generally, another plant is added to modify intensity, duration and effects of beta-carbolines. This natural additive varies according to the region and contains predominantly active principles, rich in tryptamine's metabolites, such as Psychotria viridis which contains mainly diméthyltryptamine (DMT) with hallucinogenic properties. The use by the Indians of South America of another plant, Virola, containing bderivatives of DMT will be also developed here. The direct analysis of the hallucinogenic active principles can be realized by a half and half dilution in ethanol, then detection and quantification can be realized by gas chromatography coupled with a mass spectrometer (CG/MS). Detection and quantification of carbolines in human plasma can be done by means of high-performance liquid chromatography (HPLC) with fluorescence detection after blood proteins precipitation. DMT can be determined in plasma by gas chromatography with selective nitrogen - phosphorus or mass spectrometry detection following liquid - liquid extraction with chlorobutane at alkaline pH. This carbolines and DMT association generate a deep change of the consciousness state. Only a few changes of the perception of the environment are observed, but some improvements of the general sensory functions occur. After a period of dizziness, sickness, nervousness, perspiration and sometimes violent digestive symptoms, the ingestion of ayahuasca provokes different sensory hallucinations followed by a period of tiredness; then sometimes a feverish sleep settles down, interrupted with dreams. Besides interaction with serotoninergic antidepressants, the risks related to the use of such a powerful hallucinogen as ayahuasca, are essentially psychological: it can led the mind to become destructured. It also authorizes a kind of chemical submission of the sects' followers by their leaders. That is why the French ministry of health classified the DMT and its by- products as narcotics.