Using purpose built tonometers, the carbon dioxide (CO2) dynamics in whole blood and plasma of the Antarctic fish Pagothenia borchgrevinki was determined. The tonometers maintained samples of blood at a constant temperature and blood gas concentrations, while other parameters could be altered such as the addition of hormones noradrenalin, acetazolamide and sotalol. Carbon dioxide dissociation ... curves were determined from pooled blood samples under oxygenated and deoxygenated conditions. Using an in vitro radioisotopic CO2 excretion assay, radiolabelled bicarbonate ions were injected and their rate of dehydration to CO2 was followed on blood and plasma equilibrated under an N2 atmosphere then rapidly oxygenated with air in the presence of noradrenalin, acetazolamide and sotalol. A second experiment, the dehydration rate of bicarbonate was determined on blood and plasma equilibrated under 0.5% CO2 atmosphere in air in the presence of noradrenalin. In the blood of fish, carbon dioxide is transported as bicarbonate and then converted to CO2 for release at the gills.