Often-times the information available does not permit us to assert positively that a toxic or narcotic plant employed in a primitive society is used because of hallucinatory effects. In other instances, the data at hand seem to indicate that the use is for the purpose of inducing hallucinations, yet we know of no psychotomimetic constituent in the plant. In a few cases, both the utilization of a plant for psychoactive effects and its chemical composition are equally doubtful or uncertain. Believing that the purposes of future investigations are better served by including rather than excluding these borderline examples, we have set them apart in a section of the book separate from the well known, adequately understood hallucinogenic plants, the use of which is based definitely upon the search for visual and/or other types of hallucinations. In the section of this book dealing with plants of doubtful hallucinogenic properties, no chemical formulae are given. The constituents of such plants are mentioned only by their names, while reference to their chemistry is made in bibliography. Chemical compounds with nonspecific hallucinogenic activity (e.g. ibogaine), of which the hallucinogenic property is only a side effect of other main pharmacological activities, are characterized by their structural formulae, but without description of their synthesis, which is referred to only in the bibliography. Detailed chemistry will be reported only for the specific hallucinogens.