Lipopolysaccharides (LPS) bind to Toll-like receptor 4 (TLR4), thus inducing an inflammatory cascade in immune cells as well as in cardiomyocytes. LPS pre-treatment is known to reduce the size of an ischemia-reperfusion injury (IR) in the heart. However, the signalling cascade by which LPS reduces IR is not known. Therefore coronary artery occlusion was performed in a closed-chest model on mice with (C3H/HeN, C57BL/6) and without TLR4 signalling (C3H/HeJ, TLR4-/-). One week after instrumentation, animals were stimulated with LPS (1 mg/kg BW) or PBS 16 h ahead of ligation of the left anterior descending artery. As LPS is known to increase expression of Hypoxia inducible factor 1a (HIF1a), we investigated HIF1a expression and HIF1 target genes in the myocardium 6 and 16 h after LPS challenge. LPS-pretreatment decreased infarct area by about 50% in animals with TLR4 signalling. Reduced infarct areas were also detected in all animals without TLR4 signalling. 6 h after LPS challenge HIF1a protein as well as its target genes iNOS and adrenomedullin were up-regulated only in hearts with TLR4 signalling. At the onset of ischemia iNOS and adrenomedullin were still high in those. In hearts without TLR4 HIF1a protein as well as its target genes were low independent of LPS stimulation. Thus we conclude that up-regulation of HIF1a is not necessary for a reduction of infarct size.