The results of a field study carried out in August and September 1988, in the northern Peruvian Andes are described. The area of investigation extends from Ayabaca City (about 2900 m above sea level) to Haughtiness Lagunas (about 3800 m above sea level) in the Ayabaca District, Department of Piura. This is the first time that this location has been the subject of an ethnobotanical investigation. We have collected 46 plant species, belonging to 20 families, used in the treatment of various diseases. For each plant, we report the common/local names, the crude drug formulation, method of preparation, dosage and claimed toxicity. The disease concept of this Andean population concerning the “hot” and “cold” aspects of diseases and the plants to treat them, is also discussed. Very important appear to be the use and knowledge of psychoactive plants, in particular “cimoras,” Brugmansia and Trichocereus species.