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Summary The respiratory effects of low pH were studied in the field and laboratory. The thickness of gill epithelium was similar in perch from an acid lake to that in perch from a neutral lake. This observation suggests that gill hypertrophy was not the primary cause of death of fish in the acid lake. A 24-h acid exposure to pH 5 did not affect the blood respiratory properties and ion concentrations of rainbow trout at all. However, when the acid-exposed fish were further subjected to hypoxia, the red cell water content and chloride concentration increased, red cell ATP concentration decreased, and plasma so­ dium and chloride concentration decreased. These changes were not seen in animals maintained in neutral water and subjected to hypoxia. The loss of ions may be caused by hypoxia-induced increase in the diffusion area of the gills, and hyperventilation, which initially increases the diffusion gradient across gill epithelium.