The water balance in Atlantic salmon (Salmo salar L.) overwintering in sea water (34 ‰) was investigated. With a decrease of temperature from 5.6 to 1.0°C the drinking rate decreased from 13.9 to 5.7 ml/kg/day, and the absolute amount of water absorbed decreased from 8.9 to 5.0 ml/kg/day. A decrease in temperature led, however, to an increase in the proportion of water absorbed in the intestines from 60 to 96%. Blood serum osmolarity increased from 320 to 440 mosm/1 with decreasing temperature and there was a reduction in tissue water content from 75 to 69% The disturbance of water balance at low temperature may be one of the factors responsible for mortality of salmon overwintering in sea water.