In vitro experimentation determined that red cell swelling is a consequence of activation of a Na+/H+ antiporter located on the red cell membrane. The antiporter is controlled by a ?-adrenoceptor sensitive to adrenaline located on the cell membrane, therefore these cells are sensitive to circulating stress hormones. The dynamics of red blood cell release and uptake by the spleen and the control of ... red blood cell swelling from the effects of stress was investigated in two species of Antarctic fish, Pagothenia borchgrevinki and Trematomus bernacchii. Using heat as a stressor (10°C for 10 minutes), ventilation rate, heart rate and blood pressure responses to the stressor was determined. Blood values for pH, haematocrit, haemoglobin, mean cell haemoglobin content, chloride and osmolality was determined as well. Effects on the spleen were determined and the concentration of the two stress hormones adrenaline and noradrenalin were measured.