Rainbow trout (Oncorhynchus mykiss) exposed to an acute heat shock (1h at 25°C after raising water temperature from 13°C to 25°C over 4h) mount a significant catecholamine response. The present study investigated the proximate mechanisms underlying catecholamine mobilization. Trout exposed to heat shock in vivo exhibited a significant reduction in arterial O 2 tension, but arterial O 2 concentration was not affected by heat shock, nor was catecholamine release during heat shock prevented by prior and concomitant exposure to hyperoxia (to prevent the fall in arterial O 2 tension). Thus, catecholamine mobilization probably was not triggered by impaired blood O 2 transport. Heat-shocked trout also exhibited an elevation of arterial CO 2 tension coupled with a fall in arterial pH, but these factors are not expected to trigger catecholamine release. The changes in blood O 2 and CO 2 tension occurred despite a significant hyperventilatory response to heat shock. Future studies should investigate whether catecholamine mobilization during heat shock in rainbow trout is triggered by a specific effect of high temperature activating the sympathetic nervous system via a thermosensitive transient receptor potential channel.