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Cronic exposure to an increased temperature (e.g., 10 °C as compared to 5 °C) and to a different oxygen tension can dramatically affect muscle cellularity in Atlantic salmon embryos at a developmental stage close to hatching. Maximal activities of enzymes of aerobic and anaerobic metabolism have been shown to vary with temperature and growth rates in fish but only limited data are available for embryonic fish. In order to obtain data on Atlantic salmon embryos and to be able to compare temperature and oxygen effects, the maximal activities of lactate dehydrogenase (LDH) and cytochrome c oxidase (CCO) were determined in body tissue containing mainly muscle as measures of anaerobic and aerobic metabolism, respectively. Temperatures of 5 and 10 °C, different O2tensions (50% normoxic, normoxic, 150% normoxic), and growth within (chorionated) or without (dechorionated) the egg capsule were chosen as environmental conditions. Temperature affected CCO activities and thus the CCO/LDH ratio in dechorionated but not in chorionated embryos (5 < 10 °C) and had no effect on LDH activities. However, changes in oxygen availability had an effect on all parameters measured. Tissue protein concentration increased after dechorionation but no temperature effect was found. Both LDH and CCO activities undergo complex responses to oxygen availability depending on incubation temperature. There thus appeared to be an interaction of temperature and oxygen availability with regard to maximum activities of key enzymes of the aerobic and anaerobic metabolism in the salmon embryos.