This PhD-project aims at describing how environmental factors including temperature, hypercapnia and hypoxia affect aspects of the physiology of air-breathing fish, such as ion-regulation, gas-exchange and metabolism to potentially identify physiological mechanisms underlying the evolution of air-breathing in fish. Initial research has included studies on pH_e-regulation during hypercapnia and during exposure to NO₂^- in Pangasianodon hypophthalmus. It has further included studies of blood O₂ binding properties in P. hypophthalmus and Monopterus albus and the underlying allosteric regulation of hemoglobin O₂ binding (see publications). Future studies will focus on the effect of temperature and exercise on regulation of blood gases and pH. They will also include a study on the metabolic consequences of combined hypercapnia and hypoxia acclimation.