To examine cardiorespiratory plasticity, cardiovascular function, oxygen consumption, oxygen delivery and osmotic balance were measured at velocities up to critical swimming speed (Ucrit) in seawater-adapted chinook salmon. We used two groups of fish. The control group had swum continuously for 4 months at a low intensity (0.5 BLs(-1)) and the other was given a high-intensity training regimen (a Ucrit swim test on alternate days) over the same period of time. Compared with available data for other salmonid species, the control group had a higher maximum oxygen consumption (MO2max; 244 micromol O2 min(-1) kg(-1)), cardiac output (Qmax; 65 ml min(-1) kg(-1)) and blood oxygen content (CaO2; 15 ml O2 dl(-1)). Exercise training caused a 50% increase in MO2max without changing either Ucrit or CaO2, even though there were small but significant increases in hematocrit, hemoglobin concentration and relative ventricular mass. During swimming, however, exercise-trained fish experienced a smaller decrease in body mass and muscle moisture, a smaller increase in plasma osmolality, and reduced venous oxygen stores compared with control fish. Consequently, exercise training apparently diminished the osmo-respiratory compromise, but improved oxygen extraction at the tissues. We conclude that the training-induced increase in MO2max provided benefits to systems other than the locomotory system, such as osmoregulation, enabling trained fish to better multitask physiological functions while swimming. Furthermore, because a good interspecific correlation exists between MO2max and arterial oxygen supply (TO2max; r2=0.99) among temperate fish species, it is likely that CaO2 and Qmax are principal loci for cardiorespiratory evolutionary adaptation but not for intraspecific cardiorepiratory plasticity as revealed by high intensity exercise training.