Hypoxic pulmonary vascular structural remodeling (HPVSR) is the important pathologic basis of hypoxic pulmonary hypertension (HPH). The discoveries of endogenous gaseous messenger molecules, nitric oxide (NO) and carbon monoxide (CO), have been moving the research of HPVSR to a very new phase. But the effect and significance of heme oxygenase (HO)/CO on the development of HPVSR have not been fully understood. In this study, we observed the alteration of endogenous HO/CO system in five time points during 14 days and found that the content of CO in lung homogenates in rats with HPVSR increased in a time-dependent double-peak manner. Exogenous supply of ZnPP-IX, an inhibitor of HO-1, decreased the content of CO in lung homogenate, decreased the expression of Fas and apoptotic cells in pulmonary artery smooth muscle cells (PASMCs), up-regulated the expression of PCNA in PASMCs, and worsened HPH and HPVSR of hypoxic rats. Meanwhile, exogenous supply of CO played an adverse action. The results showed that the up-regulation of HO/CO exerted a protective role in the development of HPVSR. (C) 2003 Elsevier Science (USA). All rights reserved.