Although it has been known for many years that hallucinogenic drugs are used by witch doctors in the Peruvian jungles for therapeutic purpose, there have been no descriptions of their use. The authors studied one type of treatment carried out in groups of about 10 to 12 'patients', where the healer makes a magical diagnosis and provides a potion, usually ayahuasca (active hallucinogenic agent: harmine) which is drunk by the witch doctor, his assistant and the patients. The groups meet once a week in the open air in the jungle, at dawn and for an average of 5 h. The patient attends the weekly sessions until he feels well enough to leave, and only then does he pay a voluntary fee. One of our main tasks was to study the group processes.