The European seabass is an active euryhaline teleost that migrates and forages in waters of widely differing salinities. Oxygen uptake (M-O2) was measured in seabass (average mass and forklength 510 g and 34 cm, respectively) during exercise at incremental swimming speeds in a tunnel respirometer in seawater (SW) at a salinity of 30 parts per thousand and temperature of 14 degrees C, and their maximal sustainable (critical) swimming speed (U-crit) determined. Cardiac output (Q) was measured via an ultrasound flow probe on their ventral aorta. The fish were then exposed to acute reductions in water salinity, to either SW (control), 10 parts per thousand, 5 parts per thousand, or freshwater (FW, 0 parts per thousand), and their exercise and cardiac performance measured again, 18 h later. Seabass were also acclimated to FW for 3 weeks, and then their exercise performance measured before and at 18 h after acute exposure to SW at 30 parts per thousand. In SW, seabass exhibited an exponential increase in M-O2 and Q with increasing swimming speed, to a maximum M-O2 of 339 +/- 17 mg kg(-1) h(-1) and maximum Q of 52.0 +/- 1.9 ml min(-1) kg(-1) (mean +/- 1 SEM; n=19). Both M-O2 and Q exhibited signs of a plateau as the fish approached a U-crit of 2.25 +/- 0.08 bodylengths s(-1). Increases in Q during exercise were almost exclusively due to increased heart rate rather than ventricular stroke volume. There were no significant effects of the changes in salinity upon M-O2 during exercise, U-crit or cardiac performance. This was linked to an exceptional capacity to maintain plasma osmolality and tissue water content unchanged following all salinity challenges. This extraordinary adaptation would allow the seabass to maintain skeletal and cardiac muscle function while migrating through waters of widely differing salinities.