Intracellular/extracellular and extracellular/sea-water bicarbonate exchanges were measured in Larger Spotted Dogfish (Sycliorhinus stellaris) exposed to 10 degrees C temperature step changes in a closed sea-water recirculation system. Changes of the bicarbonate concentration in blood plasma (= extracellular space) and in the recirculating sea-water were monitored for 36 h after the temperature change. Intracellular/extracellular transfer of bicarbonate was computed from bicarbonate changes in the extracellular space and sea-water. When the temperature was changed from 10 to 20 degrees C a signigicant transfer of bicarbonate was observed from the intracellular to the extracellular compartment and from the extracellular compartment to the sea-water. These transfers were reversed when the temperature was lowered from 20 to 10 degrees C. The exchange processes were practically completed after 18 h. The amount of bicarbonate exchanged between intracellular and extracellular compartments and sea-water was larger than predicted on the basis of in vitro buffer values of white, red and heart muscle, suggesting that additional tissues exchange significant amounts of bicarbonate with the extracellular space. It is concluded that physicochemical buffering is not sufficient to account for the observed adjustment of intracellular and extracellular pH and that bicarbonate exchange between body compartments and environment may be the most important regulatory mechamism, responsible for the final adjustment of acid-base balance in dogfish.