The transcription factor hypoxia inducible factor (HIF) plays a key role in orchestrating the cellular response to hypoxia. HIF activity is regulated by two families of hydroxylase enzymes, prolyl hydroxylase domains (PHD) and factor inhibiting HIF (FIH1). We have recently found that mice with a null mutation in the FIH1 gene (FIH1-/-) are protected from the reduction in cardiac contractility seen in wild type (WT) control mice during chronic hypoxia. Here, we investigate the response of the FIH1-/-hearts to ischaemia/reperfusion (IR). Hearts from 11 FIH1-/- mice and 10 WT litter-mate control mice (aged 10-12 months) were isolated and perfused in Langendorff mode with 11 mM glucose and 0.4 mM palmitate. Following 30 minutes baseline perfusion, ischaemia was induced by reduction of coronary flow rate to 0.5 ml/min/gww for 30 minutes, followed by 30 minutes reperfusion. Cardiac function was determined via water filled balloon inserted into the left ventricle of the heart, attached to a pressure transducer, expressed as rate pressure product (RPP, heart rate x left ventricular developed pressure). Rates of glycolytic flux were determined by 3H glucose labelling and net lactate efflux determined in timed perfusate samples. A further group of WT and FIH-/-hearts (n = 7 & 5 respectively) were perfused with radiolabeled (9, 10-3H) palmitate to determine fatty oxidation rates. IR significantly impaired cardiac function in both groups. Recovery of function in FIH1-/- hearts was 1.4 fold greater than WT controls (proportion RPP recovery 78.2 ±4.90 % vs. 54.2 ±9.82 % (mean ±SEM, p = 0.0496 independent t. test)). Glycolytic flux was significantly greater in FIH1-/-hearts than WT during baseline (1.180 ± 0.06 vs. 0.908 ± 0.06; p = 0.0323) and reperfusion (0.832 ± 0.10 vs 0.528 ± 0.04; p = 0.0468), with no difference in glycolytic flux during ischaemia. Furthermore, the rate of net lactate efflux over the course of perfusion was greater from FIH-/- hearts than WT (two-way ANOVA, main effect of genotype p = 0.0268). There was no difference in the rate of fat oxidation between WT and FIH-/-hearts. In summary, the present data suggest that deletion of FIH1 is cardioprotective in the face of IR. FIH1 loss is associated with accelerated glycolytic flux and increased net lactate efflux from the heart. These data are the first to suggest that modulation of FIH1 activity may provide an avenue for therapeutic intervention in the ischaemic heart.