The effects of very brief and recurrent coronary occlusions on myocardial regional shortening and its ultrastructure have been analysed. Ultrasonic crystals were implanted in the left ventricular subendocardium of 23 anaesthetized dogs with the thorax open, to measure the shortening fractions of an ischaemic and a control segment. Twenty 2 min total occlusions were provoked in the left anterior descending coronary artery, with 3 min recovery intervals (reperfusion) between occlusions. The shortening fraction decreased progressively with each occlusion, reaching a value 18.9% lower than the basal after the last ischaemic episode (P < 0.05); 32.3% after 4 h of reperfusion (P < 0.01), and 28.6% after 24 h (P < 0.01). Qualitative and quantitative ultrastructural analysis showed an increase in the mitochondrial volume of the ischaemic tissue (158% vs control, P < 0.001) with significant damage to the cell components (7.7-fold increases vs control mitochondria). These results show that when the myocardium is subjected to very brief and repeated coronary occlusions, there is progressive deterioration of systolic function with structural alterations, mainly at the mitochondrial level. These modifications are still observable 24 h after the end of ischaemic stimulation and could be the cause of transitory and/or chronic systolic dysfunctions in the absence of previous heart attack.