Picietl, peyotl, teonanactl, and ololuihqui - these were the four great divinatory plants of Mexico at the time of the Conquest. We give the names in Nahuatl, the lingua franca of that time, spoken as a mother tongue by the Aztecs and many other peoples. By 'divinatory' we mean plants that served in Middle American cultures as keys to knowledge withheld from men in their normal minds, the keys to Extra-sensory Perception, the Mediators (as the Indians believed) between men and their gods. These plants were hallucinogens, psychotropic agents, psychotomimetics, if we must use the only words of contemporary science.