This book gives a concise account of the physiology and form of the fish circulatory system. The emphasis is primarily on function, but details of structure have been included. Following a revision of ideas on hemodynamics, attention is focused on the heart as the primary pump in the fish circulatory system. The fine structure and the electrical and ionic events of cardiac myocytes are described and the major events of the cardiac cycle are outlined. The structure of the peripheral vessels then follow and attention is devoted to the circulation in certain special areas such as the gills, the renal portal system, and the secondary blood system. There are also chapters devoted to the blood and the hemopoetic tissues and an account is given of the different types of retial systems that concentrate oxygen or heat in various parts of the body. Following a description of the autonomic nervous system, the circulatory responses to exercise and hypoxia are described. The book concludes with a discussion on the circulation of hagfish and how it illuminates our understanding of the functional and structural evolution of the circulatory system.