This review systematically covers the current literature in physiology. It is designed mainly for those engaged in teaching and research and for those whose background knowledge of physiology is already well advanced. Substantially the same subjects are surveyed as in previous annual reviews, thus permitting the reader to obtain through the eyes of recognized experts, at a somewhat advanced level, a report of the principal advances in most of this field throughout the last one or two years. Each discussion is followed by a citation of the literature from which it was derived. The use of explicit models in which the significant variables are formulated algebraically appear more frequently than in previous reviews. The subjects covered in this review are Physiology and Medicine, a Transition Period, Nuclear Function, Reproduction, Development, Kidney, Water and Electrolytes, Muscle, Blood Pigments, Respiration, Heart, Circulation, Blood Volume, Alimentary Tract, Nervous System and Endocrine Glands.