The effects of low-intensity, prolonged swimming on functional recovery of the rat heart (Langendorff preparations) from ischaemia-reperfusion (I/R) were investigated. Three groups of rats (120 days old) were used: sedentary rats (S) and rats exercised by a single bout of swimming lasting 5 (E5) or 8 h (E8), respectively. The effect of exercise on the response to I/R was related to an index of oxidative damage such as lipid peroxidation, as well as to the tissue antioxidant capacity and the response of heart tissue to in vitro oxidative stress. The intrinsic performance of E5 Langendorff preparations paced at 220 beats min-1 was also determined. A group of sedentary animals was used for H2O2-treated preparations. The effect of antioxidant treatment on inotropic recovery during reperfusion was studied on preparations from 5 or 8 h swimming vitamin E-treated (EVT5 and EVT8) and 5 or 8 h swimming untreated (EVU5 and EVU8) rats. Hearts from exercised animals displayed a reduced preischaemic inotropism, which in E5 rats was accompanied by an increase in the intrinsic heart rate. The lower intrinsic cardiac inotropism of E5 animals was confirmed in the paced preparations. The reduced contractility found in control hearts after addition of H2O2 to perfusion medium suggested that the low inotropism of E5 and E8 hearts was due to an exercise-induced increase in reactive oxygen species. Inotropic recovery during reperfusion was low in the S hearts, was significantly increased in the E5 hearts, and was again reduced to the S level in the E8 hearts. In the E8 hearts the indexes of cellular damage (LDH release) and oxidative stress increased, and antioxidant capacity decreased, while in E5 hearts there was no evidence of significant changes in such parameters. Performance and reperfusion recovery of hearts from 5 h swimming rats was not affected by vitamin E treatment, while those of hearts from 8 h swimming rats was the highest observed. We suggest that the higher inotropic recovery during reperfusion in the hearts from the E5 rats is related to the negative inotropic effect of exercise. The fall in recovery following the 8 h exercise was instead related to the increased oxidative stress. Experimental Physiology (2001) 86.3, 341-348.