This book reviews the respiratory function of vertebrate red cells. I have defined the phrase "respiratory function" broadly to include, in addition to the actual oxygen and carbon dioxide transport, erythropoiesis, haemoglobin synthesis, red cell structure, the deformability of red cells in circulation, ion and substrate transport across the cell membrane, cellular metabolism, and control of cellular volume and pH. All of these aspects of the red cell function may affect gas transport between the respiratory epithelia and the tissues. Throughout the book, I have tried to relate our current knowledge about the nucleated red cell function to the wealth of information about the function of mammalian red cells. However, whenever possible, I have placed the emphasis on the nucleated red cell function for two reasons. First, the erythro­ cytes of 90% of vertebrate species are nucleated, and, second, nucleated red cell function has not been reviewed earlier in a single volume. This being the case, I have tried to make the reference list as complete as I could with regard to nucleated red cells. I hope that the approach adopted is useful for both com­ parative and human physiologists. Many people have contributed to the making of this book directly or in­ directly. Antti Soivio started me in this field. Prof. Henrik Wallgren has always encouraged fresh scientific ideas in his department. My present ideas of red cell function have been influenced by work carried out with Prof. Roy E.