Induction of the "delay phenomenon" by chronic ischemia is an established clinical procedure, but the mechanisms confering tissue protection are still incompletely understood. To elucidate the role of he-me oxygenase-1 (HO-1, HSP32) in delay, we examined in the skin-flap model of the ear of the hair-less mouse, (i) whether chronic ischemia (delay) is capable to induce expression of HO-1, and (ii) whether delay-induced HO-1 affects skin-flap microcirculation and survival by either its carbon mono-xide-associated vasodilatory action or its biliverdin-associated anti-oxidative mechanism. Chronic i-schemia was induced by transsection of the central feeding vessel of the ear 7 days before flap creati-on. The flap was finally rised by incision of 4/5 of the base of the ear. Microcirculatory dysfunction and tissue necrosis were studied by laser Doppler fluxmetry and intravital fluorescence microscopy. HO-1 protein expression was determined by Western blot analysis. Seven days of chronic ischemia (delay) induced a marked expression of HO-1. This was paralleled by a significant improvement (p<0.05) of microvascular perfusion and a reduction (p<0.05) of flap necrosis when compared with non-delayed controls. Importantly, blockade of HO-1 activity by tin protoporhyrin-IX (SnPP-IX) completely blunted the protection of microcirculation and the improvement of tissue survival. Additional administration of the vitamin E analogue trolox after blockade of HO-1 to mimic exclusively the anti-oxidative action of the heat-shock protein did not restore the HO-1-associated microcirculatory improvement and only transiently attenuated the manifestation of flap necrosis. Thus, our data indicate that the delay-induced protection from tissue necrosis is mediated by HO-1, predominantly through its carbon monoxide-associated action of adequately maintaining nutritive capillary perfusion.