This 1978 volume contains papers from contributors to the Third International Conference on Comparative physiology. The Conference selected particular areas for examination. In the first section of this volume the problems of how animals can take up water vapour from the atmosphere are considered as well as advances in studies of how water movements across epithelia are generated by solute movements. The second section deals with how a wide variety of animals, both invertebrate and vertebrate, living under stress in ionically unbalanced environments cope with the unusual difficulties of ionic regulation. In the final section biologists and physicists examine the role of fluid mechanics in biology. Both the theoretical basis of the hydrodynamics and aerodynamics and the biological investigations on the variety of fluid flows encountered inside and around organisms are presented.