The renin-angiotensin system and the mechanisms regulating this system developed during the adaptive evolution of verte brates, along with many other systems involved in the in tegrated survival of the organism. Because animal species have evolved from common ancestral populations, a basis for the comparison of body structures and physiological processes ex ists among animal groups belonging to different classifications. The comparative approach provides a better understanding of the structure and function of adaptive systems and facilitates the development of general principles governing these systems among animal groups; further, this approach reveals significant characteristics specific to certain animal groups. As the evolu tion of adaptation of animals to environmental conditions is explored, directions for future research are suggested. In this book, advances in research on the renin-angiotensin system are described with emphasis on the comparative aspects. However, since studies on the renin-angiotensin system of birds, reptiles, amphibians and fishes are limited compared with those con ducted in mammals, in some chapters descriptions are con cerned primarily with mammals. It has taken a long time to write this volume, and the topic is a broad one, with new data always emerging; therefore, certain aspects, and sometimes the most recent information, may not be included. Chapters 1-3 and sections 8. 1-8. 4,8. 6, 8. 7 were written by H. K. ; Chapters 4-7 and Section 8. 5 by Y. T. ; Chapter 9 was written by both authors. H. Uemura, M. Nozaki, Y. Okawara, We are indebted to Drs.