The impact ofin vivoischaemia and ischaemia‐reperfusion (I‐R) on mitochondrial respiratory function was investigated in hypertrophied (HP) hearts with aortic constriction compared with control hearts using an open‐chest rat surgical model. Moreover, mitochondrial susceptibility to superoxide radicals (O2P‐)in vitrowas examined in HP and control hearts with or without I‐R. With the site I substrates pyruvate‐malate, mitochondrial state 4 (basal) respiration and the respiratory control index (RCI) were not affected by either ischaemia alone or I‐R in both HP and control hearts. State 3 (ADP‐stimulated) respiration was increased with I‐R in control hearts, but showed a reduction after I‐R in the HP hearts. Exposure of mitochondria to O2P‐(20 nmhypoxanthine in the presence of 0.13 unit mL‐1xanthine oxidase) significantly increased state 4 respiration, whereas state 3 respiration and RCI were decreased in all treatment groups. I‐R hearts in both HP and control showed greater increases in state 4 respiration with O2P‐than either sham or ischaemic hearts. HP hearts exhibited a significantly lesser extent of inhibition in state 3 respiration and RCI by O2P‐compared with control hearts. These changes in mitochondrial respiratory properties were not observed with the site II substrate succinate. Myocardial reduced vs. oxidized glutathione ratio was significantly decreased after I‐R in both control and HP hearts. Malondialdehyde content showed an increase with I‐R, but the increase was significant only in control hearts. These data indicate that short‐termin vivoI‐R does not impair heart mitochondrial respiratory function, but renders the organelles more vulnerable to imposed oxidative stress. Mitochondria from the HP hearts are more resistant to free radical damage under normal and ischaemic conditions; however, this advantage is s