This book constitutes the proceedings of a NATO Advanced Research Workshop held at Chateau de Bonas (France) from 10-15 July 1990 on the Midbrain Periaqueductal Gray Matter (PAG). The aim of this meeting was to review and integrate our knowledge about the functional, anatomical and neuro chemical organization of the PAG. The PAG has been the subject of many investi gations during the last decade usually on different topics (e.g., pain modulation, defensive and sexual behavior) and generally there has been little interchange between the different research areas. The main purpose of this meeting was to bring together, for the first time, scientists who have worked on the PAG from different perspectives. This book does not pretend to present an exhaustive review of the data collected during the last 20 years of research on the PAG. The contributors to this book have been selected because their data provide key elements in the search to understand both the organization of the PAG and the role of this structure in the integration of behavior. We believe that this book will provide clues that will assist in unraveling the organization of the PAG in the coming years.