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ABSTRACT Cardiac and behavioural responses to hypoxia and warming were investigated in free-swimming gilthead seabream Sparus aurata equipped with biologging tags in the peritoneal cavity. After suitable recovery in a holding tank, heart rate ( f H ) and the variance of tri-axial body acceleration (VAR m ) were logged during exposure to stepwise progressive hypoxia or warming, comparing when either swimming in a tank or confined to individual respirometer chambers. When undisturbed under control conditions (normoxia, 21 °C), mean f H was significantly lower in tank than respirometers. In progressive hypoxia (100 - 15% oxygen saturation), mean f H in the tank was significantly lower than respirometers at oxygen levels until 40%, with significant bradycardia in both holding conditions below this. Mean VAR m was low and invariant in hypoxia. Warming (21 to 31 °C) caused progressive tachycardia with no differences in f H between holding conditions. Mean VAR m was, however, significantly higher in the tank during warming, with a positive relationship between VAR m and f H across all temperatures. Therefore, spontaneous activity contributed to raising f H of fish in the tank during warming. Mean f H in respirometers had a highly significant linear relationship with mean rates of oxygen uptake, considering data from hypoxia and warming together. The high f H of confined S. aurata indicates that static respirometry techniques may bias estimates of metabolic traits in some fish species. Biologging on free-swimming fish revealed novel information about cardiac responses to environmental stressors, which may be closer to responses exhibited by fish in their natural environment. SUMMARY STATEMENT Implantable biologgers were used to provide the first measurements of cardiac responses to hypoxia and warming in a free-swimming fish, revealing that confinement in respirometer chambers raises heart rate, with consequences for estimates of metabolic rates.