Aims: To investigate altered states of consciousness (ASCs) and short-term psychological outcomes caused by the first ritual use of ayahuasca in the religious groups União do Vegetal and Santo Daime, and to investigate the association of these phenomena to previous individual variables (set) and to the variables of the context of the hallucinogen use (setting). Methods: 28 subjects were evaluated 1-4 days before the first ayahuasca experience (T0) and 7-2 weeks after this experience (T1). The motives/expectancies to try ayahuasca, the ASCs, and specific psychosocial, behavioral e biographic aspects were assessed by semi-structured qualitative interviews. Minor psychiatric symptoms were assessed before and after the experience by the scale of clinical impression CIS-R. Results: Although sharing various aspects with hallucinogens, the ASCs induced by the ritual use of ayahuasca seem to produce some unique effects. Visual phenomena, numinosity, peacefulness and religious and biographical insights were the most frequent ASCs experiences. A significant reduction of the intensity of psychiatric symptoms occurred in the Santo Daime group after the hallucinogen experience. Subjects of both groups related behavioral changes towards assertivity, serenity and vivacity/joy. Conclusions: It is suggested that the configuration of ASCs results from a combination of set and setting aspects and structural consciousness alterations induced by the psychoactive beverage. Some hypotheses concerning the behavioral and emotional changes post experience are also suggested: processes related to religious conversion; processes related to relieves and satisfactions caused by ruptures of day-to-day monotony, and suggestive processes that involve set and ritual setting aspects.