Cardiac function has been hypothesised to be a primary factor influencing swimming performance in teleost fish. Using the in situ perfused heart (Farrell) set-up we examined cardiac function in cod acclimated to 0, 4 and 10°C. Only small differences in maximal cardiac output (Qmax) and the maximal myocardial power (max PO) tests were seen between 4 and 10°C acclimated cod with regards to cardiac output (CO) and maximum power generated, and these were primarily due to significant chrontropic rather than inotropic effects. In both the Qmax and max PO tests, the cardiac output was significantly lower in the 0°C cod compared to the 10°C cod. Again this was due to significant chronotropic effects. Through acute temperature reversal, i.e. 10°C cod at 4°C and the 4°C cod at 10°C we found CO to be significantly larger in 4°C cod at 10°C compared to 10°C cod at 10°C, again due primarily to a significant increase in HR during the Qmax test, although SV also tended to be greater. Again the CO and maximum power generated during the max PO test were significantly elevated, however this was due to alterations in both SV and HR. It appears that heart rate alone is responsible for regulation of CO and power generation in cod when challenged at their acclimation temperatures, while SV remains constant. During the acute temperature change it seems that both the HR and SV are altered to maintain and in fact increase CO.