Schmeda-Hirschmann, G. Universidad de Talca, Departamento de Ciencias Biológicas, Casilla 747, Talca, Chile.The Ayoreo is a hunter-gatherer tribe of Amerindians which occupy the central-northern part of the Paraguayan Chaco. The whole Ayoreo culture cannot be disassociated from religious beliefs. Disease is considered of supernatural origin and as the result of breaking or disobeying the tabu which regulates existence. A description of the shamanic practices is given to understand better the position of health practices in the Ayoreo culture, particularly the use of medicinal and hallucinogenic plants. Fragments of the Asojna ritual and the methods for becoming shaman; the initiation of the last living shaman, as well as references to the magic powers of the shaman are presented. Diagnosis and treatment included invocations to plant and animal spirits and the use of a few medicinal plants. The plants used as medicine or invoked for healing are presented for the first time. Of particular interest is the identification of two Euphorbiaceae as ritual plants by the Ayoreo.