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Aims Cardiac troponin T (cTnT) and troponin I (cTnI) are expressed as an obligate 1:1 complex in the myocardium. However, blood levels of cTnI often rise much higher than that of cTnT in myocardial infarction (MI), whereas cTnT is often higher in patients with stable conditions such as atrial fibrillation. Here we examine high-sensitive (hs) cTnI and hs-cTnT after different durations of experimental cardiac ischaemia. Methods and results hs-cTnI, hs-cTnT, and the hs-cTnT/hs-cTnI ratio were measured in plasma samples from rats before and at 30 and 120 min after 5, 10, 15, and 30 min of myocardial ischaemia. The animals were killed after 120 min of reperfusion, and the infarct volume and volume at risk were measured. hs-cTnI, hs-cTnT, and the hs-cTnT/hs-cTnI ratio were also measured in plasma samples collected from patients with ST-elevation myocardial infarction (STEMI). hs-cTnT and hs-cTnI increased over 10-fold in all rats subjected to ischaemia. The increase of hs-cTnI and hs-cTnT after 30 min was similar, resulting in a hs-cTnI/hs-cTnT ratio around 1. The hs-cTnI/hs-cTnT ratio was also around 1 in blood samples collected at 120 min in rats subjected to 5 or 10 min of ischaemia where no localized necrosis was observed. In contrast, the hs-cTnI/hs-cTnT ratio at 2 h was 3.6-5.5 after longer ischaemia that induced cardiac necrosis. The large hs-cTnI/hs-cTnT ratio was confirmed in patients with anterior STEMI. Conclusion Both hs-cTnI and hs-cTnT increased similarly after brief periods of ischaemia that did not cause overt necrosis, whereas the hs-cTnI/hs-cTnT ratio tended to increase following longer ischaemia that induced substantial necrosis. A low hs-cTnI/hs-cTnT ratio around 1 may signify non-necrotic cTn release.