There are increasingly more areas where pharmacology and toxicology converge, thus allowing a better understanding of the response to drugs and their possible toxic effects. The goal of this international symposium, organized by the Esteve Foundation on October 6-8, 1986 in Majorca, was to explore some of these areas. Professors Gabriel L. Plaa, Patrick du Souich and Sergio Erill, from the Department of Pharmacology of the University of Montreal, coordinated this event and edited volume number 2 of Esteve Foundation Symposia.