When this book was published in 1971, there were several thousand papers dealing with the structure and functions of the heart and blood vessels of fishes. This monograph integrates this information into a much more coherent account of the circulatory function in fish. The structure and function of the heart, arteries, veins and capillaries are discussed and special attention is devoted to the retial capillary exchange systems. The properties of the blood of fish which fit it carry the respiratory gases in the aquatic milieu and the biochemical regulation of the capacity of haemoglobin to bond oxygen, are described. The author examines the role of the nervous system in regulating the activity of the heart and peripheral vessels and evaluates the part this plays in the response of the fish to exercise and to oxygen deficiency.