From very remote times, the indigenous inabitants of various parts of South America have been aware of the hallucinogenic properties of diverse species of the genus Piptadenia. The purpose of the present study is to bring out the salient facts concerning this botanical genus, which are of special importance today when pharmacotoxicology has reached what might be called the epoch of the "psychopharmacos ". The chief causes and motives which lead various primitive tribes to make use of powdered piptadenia seeds would appear to be the following: 1. The need for an "excitant" before battle. We have already quoted various authorities on the use of this drug by the Otomac Indians for the sole purpose of procuring an intense stimulus which incites them to kill and wound their enemies. However, the arousing of aggressive tendencies does not appear to be common to all cases. The effect of the drug depends to a large extent on the "personality" of the subject under its influence. In more pacific tribes, as is shown in the case of the Guahibos, the drug does not promote aggressiveness. 2. The need of a suitable intermediary for "sacred invocation rituals ". In this respect, Lisandro Alvarado [ 13] writes: "At the beginning of the twentieth century the most renowned among the witch doctors in the Upper Orinoco area was the Guahibo Piache from Ocuné, at the source of the River Vichada, who was known as kuragina (Good Spirit) . . . . at the end of this retreat, he takes yopo powder until he becomes terribly drunk, his intoxication provoking such violent spasms and delirium that he has to be assisted by his acolytes or relatives, and soon afterwards remains sunk in a deep lethargy..." Further on the same author tells us: "The Otomacs also had mojanes who made their prophecies preferably with the help of yopo, which was well known to them; but among these Indians, as among the Guahibos and Achaguas, yopo does not seem to have been a monopoly of the piaches used solely for prophesying purposes." 3. Use as a "vice ". The drug has been used in this way by many tribes from very remote times. The vice is encouraged particularly by: ( a) the desire for an abnormal stimulant; ( b) the desire to provoke a state of euphoria; ( c) an emulative spirit and the maintenance of a tradition; ( d) ignorance. Another aspect of the question which should not be overlooked is the association of the drug with other toxic substances, especially alcohol and tobacco. In some regions the use of this drug is also associated with that of caá-pí ( Banisteria caapí Spruce), another powerful hallucinogenic agent.