Many Antarctic fishes have taken advantage of the combination of low oxygen requirements and high availability of oxygen by reducing the number of erythrocytes and the amount of intracellular haemoglobin. A vital link in the relationship between haemoglobin concentration, oxygen availability and metabolic requirements is the affinity of haemoglobin for oxygen. However, other factors such as pH and ... ATP concentration also have significant affects on haemoglobin oxygen affinities. Oxygen affinities, pH and ATP concentrations in whole blood from a range of Antarctic fish (Pagothenia borchgrevinki, Dissostichus mawsoni, Trematomus bernacchii, T. lonnbergi and Rhigophila dearborni) was measured. Oxygen binding curves were determined, as well as the effects of pH and temperature on oxygen affinity. The levels of organic phosphate compounds which regulate oxygen affinity, were measured in vivo. In D. mawsoni, both arterial and venous oxygen tension and pH were determined. Under normal conditions, the reduced haemoglobin concentrations of Antarctic fishes can satisfy their low metabolic requirements for oxygen. An important function of haemoglobin may be as an emergency oxygen supply. How Antarctic fishes cope with unusually high metabolic loads such as the stress of heavy exercise was investigated on the blood chemistry and haematology (changes in haematocrit and haemoglobin concentrations, blood lactate, erythrocytic ATP and man corpuscular haemoglobin) of D. mawsoni and P. borchgrevinki after capture and anaesthesia and agitational stress.