Global climate change is predicted to continue with increasing average temperatures and more frequent temperature extreme events. This will have a profound impact on ectothermic animals such as fish where the body temperature and metabolism is directly determined by the ambient temperature. Current prevailing hypotheses suggest that insufficient oxygen transport is a primary limitation for ectothermic animals at elevated temperature, but knowledge about the adjustments to the cardiorespiratory system in fish during long-term warming (i.e. via acclimation and genetic adaptation) is still limited. Results from a recently launched research project on cardiovascular and respiratory function in Eurasian perch (Perca fluviatilis) that live and reproduce in the chronically heated ‘Biotest enclosure’ in the Baltic Sea off the nuclear power plant in Forsmark in Sweden will be presented and discussed. Cooling water from the nearby nuclear reactors has been directed to the 90 hectare enclosure since 1980 keeping water temperatures 5-10°C above ambient for over 30 years, but at otherwise the same abiotic conditions as the surrounding archipelago. Thus, this experimental facility presents a unique opportunity to study the physiological responses to chronic warming in a wild fish population. Preliminary results reveal that fish from the Biotest enclosure have smaller hearts relative to their body mass, and they are considerably more tolerant to acute temperature increase and better able to maintain cardiac function during acute thermal challenges. In addition, they have a depressed metabolic rate and a lower resting heart rate due to a higher cholinergic tone and a reduced intrinsic cardiac pacemaker rate relative to fish from outside the enclosure when compared at the same temperature.