Ischaemic preconditioning (IP) is a strong endogenous infarct reducing stimulus which has not previously been evaluated with myocardial perfusion imaging using 99mTc-MIBI. Factors responsible for cellular MIBI uptake are affected by both IP and acute ischaemia (plasma membrane and mitochondrial membrane potential and oxidative metabolism). IP seems to involve mitochondrial K-ATP channels affecting mitochondrial membrane potential and thereby potentially MIBI uptake. The study evaluated the performance of MPI with MIBI as a tracer to characterise the extent that severely ischaemic compromised myocardium was salvaged by IP. In a closed chest model, an ischaemic preconditioned group (8 pigs) subjected to IP before introducing a 45 min period of catheter based coronary occlusion was compared with a control group (9 pigs). Area at risk'(AAR), infarct size (IS) and IS relative to AAR was determined by MIBI SPECT and by a standard histochemical method. The results demonstrated that infarct size was significantly smaller in the IP group both relative to left ventricle (IS/LV) and to area at risk (IS/AAR). Both AAR/LV and IS/LV, however, were greater when measured by MPI than with histochemistry. There was no difference in the ratio between infarct size and area at risk (IS/AAR). In conclusion, MPI with MIBI is a reliable measurement of infarct reduction by ischaemic preconditioning. Myocardium affected by recent ischaemia is correctly distinguished as viable by MPI in early reperfusion, when compared to a standard histochemical technique.