The concept of this book has developed over the past fi fteen years as interest in the water and electrolyte disturbances associated with most environmental settings moved from a research area of descriptive discovery to one dealing with the mechanisms responsible for the previously observed disturbances. Most of the contributing authors have been involved in both aspects of this evolution of research, focusing on those problems associated with body fluid and electrolyte balance and searching for hormonal explanations. What did not accompany this transition, however, was a source of information encompassing the area of interest. Instead, the previous format of environmentally focused symposia, reviews, and books continued to be the only sources available. For instance, various books deal with the physiology of high altitude, space, or exercise but do not necessarily provide adequate coverage of water and electrolyte disturbances. To our knowledge, the format of this book is unique. We have made the central focus water and electrolyte physiology with an emphasis on endocrinology and tried to comprehensively cover this area of physiology in some of the more heavily studied environments. This book too, then, will have its limitations in coverage. For instance, in-depth coverage of the respiratory and cardiovascular responses to the high altitude en vironment will not be found, but since these areas are so integrally associated with water and electrolyte regulation they are not ignored.