Many psychoactive plants owe their activity to structures containing an indole nucleus. This nucleus is evident in the synthetic LSD and in such well known compounds as yohimbine, reserpine, physostigmine and the strychnos alkaloids. And it is to the presence of indole alkaloids that many of the most striking hallucinogenic plants owe their biodynamic activi ty: iboga in Africa (with ibogaine and related alkaloids); the sacred mushrooms (with psilocybine) and morning glories (with ergoline alkaloids) of Mexico; and, in South America, the drinks called ayahuasca (with 0-carbolines) and in vinho de jurema (with N,N-dimethyltryptamine) and the snuffs known as yopo, huilca and epena (all with various tryptamines). Indole alkaloids are not confined to plants: bufotenine (5-hydroxy-N,N-dimethyltryptamine) is found in the skin of certain toads and the biogenic amine serotonine (5-hydroxy-tryptamine) is present in minute amounts in the central nervous tissue of warm-blooded animals. There is every probability that future research will show that other narcotic preparations from both the New and the Old World are effective because of indole alkaloids. At the present time, however, we do know that many of the most important of the hallucinogenic plants of tropical America owe their activity to these compounds. We know, also, that primitive cultures in the New World have exhibited an uncanny perspicacity in discovering in such an overwhelmingly rich flora a large number of psychoactive species. And, finally, we know that undoubtedly new hallucinogemcally active chemical constituents still await discovery in the rich psychoactive flora of both the Old and the New Worlds.