A study of the use and effect of peyote and Cannabis sativa (marihuana). It is intended to reduce uncertainty surrounding benefits or perils inherent in the use of LSD and other hallucinogens. The history of peyotism among the American Indians is examined, including a detailed review of method and frequency of use, incidence of addiction and psychosis, motivation, relation to alcoholism, socioeconomic and psychological differences between peyotists and non-peyotists. A similar review of the use of cannabis in the United States and other parts of the world is presented. Data on peyote and cannabis are correlated with those on LSD in an effort to predict the future direction and accompanying problems of hallucinogens in the present culture.