Climate change may result in future alterations in thermal regime which could markedly affect the early developmental stages of cold water fish due to their expected high sensitivity to increasing temperature. In the present study, the effect of temperature increase of 2, 4 and 6°C on the oxygen consumption rate (R), the activity of respiratory electron transport system (ETS) and oxidative stress have been studied in four developmental stages of the marble trout (Salmo marmoratus)-eyed eggs, yolk-sac larvae and juveniles of 1 and 3 months. Oxygen consumption rate and ETS activity increased with level of development and with temperature in all four stages. ETS/R ratios decreased during development and correlated with temperature in eyed eggs, larvae and juveniles of 1 month, but not in juveniles of 3 months. Low ETS/R ratios at higher temperatures indicate stress response in eyed eggs, the most temperature sensitive developmental stage. Catalase (CAT) and glutathione reductase (GR) activities increased during development, but responded differently to elevated temperature in the different developmental stages. Stress in eyed eggs, caused by higher temperatures, resulted in increased oxygen consumption rate and increased activities of CAT and GR. Larvae were sensitive to increased temperature only at the highest experimental temperature of 16°C. Increased temperature did not stress the metabolism of the juveniles, since they were able to compensate their metabolic activity. The earlier developmental stages of marble trout are thus more sensitive to temperature increase than juveniles and therefore more endangered by higher water temperatures. This is the first report connecting oxygen consumption, ETS activity and ETS/R ratio with the activities of antioxidant enzymes in relation to increased temperature in salmonids.