Out of the vast array of species in the plant kingdom — variously estimated at from 200,000 to 800,000 — a few have been employed in primitive societies for millennia to induce visual, auditory, tactile, and other hallucinations. Because of their earthly effects that often defy description, they have usually been considered sacred and have played central roles as sacraments in aboriginal religions (Schultes 1969a). Scientific interest in hallucinogenic agents has recently been intense, partly because of the hope of finding potentially valuable drugs for use in experi mental or even therapeutic psychiatry and also for use as possible tools in an explanation of the biochemical origins of mental abnormalities (Hoffer & Osmond 1967). While psychoactive species are widely scattered throughout the plant world, they appear to be concentrated more or less amongst the fungi and angiosperms. The bacteria, algae, lichens, bryophytes, ferns, and gymnosperms seem to be notably poor or lacking in species with hallucinogenic properties (Schultes 1969-1970). These hallucinogenic properties can be ascribed, likewise, to only a few kinds of organic constituents, which may be conveniently divided into two broad groups: nitrogenous and non-nitrogenous compounds. In the study of hallucinogenic plants, two considerations must be borne in mind. One considera tion reminds us that tfiere are some of these psychoactive plants used in primitive societies for which the active chemical principles are as yet not known. The other emphasizes that man undoubtedly has utilized only a few of the species that actually do possess hallucinogenic principles: we are, as yet, far from knowing how many plants are endowed with psycho tomimetic constituents, but there are certainly many more than the few employed by man as hallucinogens (Schultes 1967). While almost all hallucinogenic compounds are of vegetal origin, a few may be wholly or partly synthetic. The potent hallucinogen, lysergic acid diethylamide (LSD), although very closely allied chemically to the naturally occurring ergolines, has not been found in the plant kingdom.