This report describes psychological assessments of the first time ritual use of ayahuasca in the religious groups Uniao do Vegetal and Santo Daime. Nineteen subjects who tried the beverage in Santo Daime rituals and nine subjects who tried it in Uniao do Vegetal rituals were evaluated one to four days before their first ayahuasca experience in life and one to two weeks after this experience. Semistructured interviews and a structured psychiatric scale were used in the first evaluation to elicit set variables concerning attitudes towards the ayahuasca experience and to elicit mental health status. Mental health status was reassessed in the second evaluation, which also included a semistructured interview concerning the phenomenology of altered states of consciousness (ASCs). Predominantly positive expectancies concerning the ayahuasca experience were the most prominent findings concerning set variables. Visual phenomena, numinousness, peacefulness, insights and a distressing reaction were the most salient ASC experiences. A significant reduction of the intensity of minor psychiatric symptoms occurred in the Santo Daime group after the hallucinogen experience. Subjects in both groups reported behavioral changes towards assertiveness, serenity and vivacity/joy. The set and setting hypothesis, suggestibility processes, as well as the supposed unique effects of ayahuasca are used in discussing these findings. Keywords-hallucinogens, psychiatric symptoms, religion, ritual, states of consciousness