Ischemia-reperfusion provokes barrier failure of the coronary microvasculature,leading to myocardial edema development that jeopardizes functional recovery of the heart.This is due to a Ca2+ dependent activation of the endothelial contractile machinery which in turn leads to opening of intercellular gaps.The aim of the present study was to investigate whether stimulation of cGMP- signalling by activation of soluble guanylyl cyclase with HMR1766 or DEAnonoate or particulate guanylyl cylclase with Urodilatin can reduce reperfusion induced endothelial barrier failure and protect the heart against an imminent reperfusion injury and if this is due to an influence on cytosolic Ca2+ homeostasis during reperfusion. In cultured rat coronary endo- thelial monolayers stimulation of cGMP-signalling during reperfusion markedly reduced gap-formation and attenuates the reperfusion-induced increase in cytosolic Ca2+. This is due to an augmented sequestration in ER,since stored amount of Ca2+ and SERCA activity were increased. Furthermore, MLC-phosphory- lation decreased upon GC activation.In isolated saline perfused rat hearts, reperfusion-induced increase of myocardial water content was significantly reduced.In conclusion,activation of the cGMP signalling pathway effectively protects the coronary endo- thelium against reperfusion-induced barrier failure, in cultured coronary endothelium as well as in the coronary system in situ.