The affects of acute increase in temperature on haematocrit and on the ionic and osmotic homeostasis of Pagothenia borchgrevinki was investigated. Fish were captured and placed in aquariums at a temperature of 0° for 48 hours undisturbed and unfed. Groups of six fish were transferred to aquariums heated to 10°C. After 10 minutes the fish were removed to the 0°C aquariums to recover. Each fish was sampled before the temperature stressor and immediately after, and at 30 min, 1, 2, 4, 8, 12, 24 and 48 hours of recovery. Blood samples were centrifuged to determine haematocrit and the removed plasma was analysed for chloride concentrations and osmolarity.