Stunning of the myocardium is mechanical dysfunction that persists after the re-establishment of coronary flow despite the absence of damage and the lack of any evidence for continuing ischaemia. Many mechanisms have been put forward and largely disproved. The most probable mechanism is abnormal function of the sarcoplasmic reticulum due to the generation of oxygen radicals within the myocardium. An hypothesis is proposed based on evidence indicating that the function of the calcium release channel of the sarcoplasmic reticulum is modified by reactive oxygen species before the channel is rendered non-functional.