Summary

Acid-base regulation in rainbow trout acclimated to about 3, 100 and SOOmmolP1 Na+ and Cl~, at constant water [HCO3~], was assessed during 24h of exposure to 1 % CO2 and during recovery. The respiratory acidosis induced by a rise in plasma Pco2 to about 1.15kPa (8.5mmHg, Smmoll"1), 1.33kPa (lOmmHg, lOOmmoir1) or 1.5 kPa (11.2 mmHg, SOOmmoll"1) was partially compensated for by accumulation of plasma HCO3~. The degree of pH compen-sation depended on the salinity of the environmental water, being about 61,82 and 88 % at 3, 100 and SOOmmoll"1 Na+ and Cl~, respectively. [HCO3~] in animals acclimated to 100 and SOOmmoll"1 rose to higher values than that in fish at 3mmoll~'. Plasma [Cl~] decreased during hypercapnia as compared to control concen-trations in all groups of fish. Plasma [Na+] rose during the first 8 h of hypercapnia in fish acclimated to all three salinities, but recovered towards control values during the remainder of hypercapnia. The rise in plasma [HCO3~] was signifi-cantly related to the fall in plasma [Cl~], whereas the changes in plasma [Na+