Hypoxic Pulmonary Vasoconstriction (HPV) is a mechanism for regulating pulmonary blood flow that leads to a redistribution of perfusion from hypoxic alveolar areas to better ventilated alveoli of the lung, thus ensuring an optimized oxygenation of the circulating blood. Despite intensive research, the exact oxygen sensor and the underlying signal trans-duction pathway have not yet been fully identified. However, it is already known that pul-monary vascular tone and HPV can be regulated by nitric oxide (NO/cGMP-dependent mechanisms), cytochrome P450 (CYP450)-dependent metabolites or mitochondrial signals, all of which are targets of carbon monoxide (CO). In recent years, CO has gained more importance as a possible therapeutic agent despite its well known toxic effects. Endogenously produced by hemeoxygenase through degradation of heme proteins CO is accredited with antioxidative, anti-inflammatory, antiproliferative, antiapoptotic and vasoregulatory properties. In the last two decades, this has led to the de-velopment of so-called carbon monoxide-releasing molecules (CORMs), which are capable of releasing smaller, controlled amounts of CO. In the lung, CO and CORMs appear to counteract the development of pulmonary hypertension (PH). However, the direct effect of CORMs on the pulmonary vascular reactivity and HPV in isolated lungs has not yet been investigated. In this study, the effects of 10% CO inhalation and the application of the substances CORM-2 and CORM-3 on pulmonary vascular tone during normoxia and acute hypoxia (1% O2 for 10 min) in isolated ventilated and perfused mouse lungs were assessed. The interaction of CO as well as CORM-2 with pulmonary cytochromes was detected simulta-neously by tissue spectrophotometry. This allowed conclusions to be drawn about possible binding partners of CO that could be related to the regulation of pulmonary vasoreactivity. Inhaled CO decreased both HPV and the vasoconstriction induced by the thromboxane mi-metic U-46619 but did not alter KCL-induced vasoconstriction. In contrast, high concentra-tions of CORM-2 and CORM-3, which led to a measurable release of CO in vitro, induced a continuous increase in normoxic tone, which was even augmented in the presence of erythrocytes. Inhaled CO caused spectral alterations that were compatible with the inhibi-tion of CYP450. In contrast, during application of CORM-2 spectrophotometric signs of interaction with CYP450 could not be detected. Inhaled CO at supraphysiological concentrations has a vasodilatory effect in the isolated, ventilated and perfused mouse lung and is associated with an inhibition of CYP450. Appli-cation of CORMs at high concentrations in the model of the isolated, ventilated and per-fused lung however caused a pulmonary vasoconstriction. Further investigations are neces-sary to elucidate the discrepancy between the results of inhaled CO or in vivo administered CORMs and the effects of CORMs in the isolated lung.