1. Atlantic cod (Gadus morhua L.) acclimated to a temperature of 5 °C and 30 salinity were equipped with ultrasonic transmitters which allowed continuous monitoring of their heart rate and their position in the water column. Fish were placed in a 125 m3 tower tank which permitted various environmentally relevant modifications of the salinity and oxygenation conditions. Cod physiological and behavioural responses were followed in parallel to the environmental manipulations. Some of the experimental conditions studied in the tower tank were also reproduced in a swimming respirometer where fish oxygen consumption and heart rate were monitored at various levels of activity. 2. Lowering salinity from 30 to 26 did not change resting oxygen consumption, but increased active oxygen consumption. 3. Lowering salinity from 30 to 26 increased heart rate over the whole range of swimming speeds except at maximum speed. 4. Lowering oxygen tension to 9 kPa decreased oxygen consumption over the whole range of swimming speeds and decreased resting heart rate. 5. Low salinity did not significantly affect the relationship between heart rate and oxygen consumption. 6. Low oxygen levels decreased the oxygen transported per heart beat. 7. In the tower tank, bursts of activity were associated with tachycardias. 8. In uniform conditions, fish swam more deeply during the day than at night. 9. After an exploratory period of approximately 6 h, fish chose to remain in a low-salinity upper layer of the tank. Thereafter, high salinities were avoided. Fish tended to select low salinities if a choice was provided. 10. Fish generally avoided zones of low oxygen (<9 kPa) but continued voluntarily to enter regions with values as low as 3.0 kPa for short excursions or if food was offered.