Carboxyhemoglobin (COHb) formation is triggered by the inducible isoform of heme oxygenase (HO-1) catalyzing carbon monoxide (CO) production through breakdown of heme molecules, exposure to CO or both. In the setting of CO poisoning, COHb is regarded as a reliable marker characterizing both severity of injury and efficacy of treatment strategies. This study was designed as a prospective laboratory experiment to elucidate potential interdependencies between COHb generation, oxygenation, and pulmonary shunt fraction (Qs/Qt) in an ovine model of smoke inhalation injury. Chronically instrumented ewes (n = 15) were repeatedly subjected to cotton smoke (4 x 12 breaths) according to an established protocol. This approach resulted in a progressive increase in COHb formation that was interrelated with the degree of Qs/Qt (P < 0.001) and inversely correlated with both arterial and mixed venous HbO(2) saturation (r = -0.96 and -0.93). Although the arteriovenous COHb gradient successively decreased over time, COHb determined in venous blood underestimated the arterial content. © 2003 Elsevier Inc. All rights reserved.