a branch of animal physiology that utilizes the comparative method in studying the physiological functions of various representatives of the animal kingdom. Together with the physiology of aging and ecological physiology (which studies the development of functions in relation to environmental conditions), comparative physiology constitutes an important branch of the evolutionary physiology of animals.

Comparative physiology is concerned mainly with the physiological characteristics of animals of different taxonomic groups (phyla, classes), the specific functions of organisms (respiration, digestion, nervous-system reactions), and the functions of the individual structural elements of organs, tissues, cells, and biologically active substances (hormones, enzymes, mediators, respiratory pigments). In the USSR, the development of comparative physiology as an independent field of research is associated with the work of L. A. Orbeli, Kh. S. Koshtoiants, and E. M. Kreps.