Improvements in hemoglobin-based oxygen-carrying (HBOCs) solutions have overcome the toxicities that plagued earlier efforts. However, limitations of the efficacy of HBOCs are emerging. The potential limitations of an HBOC were studied in an ovine model (n = 6) of exchange transfusion. Hemodynamic, oxygen transport, and hemoglobin kinetic parameters were examined during isovolumic blood exchange to a final hematocrit of 3.2 +/- 0.7% and a plasma hemoglobin concentration of 8.1 +/- 0.4 g/dl while sheep were awake and breathing room air. However, the infusion of HBOC was associated with immediate increases in systemic and pulmonary arterial pressures. Despite hemodilution with HBOC, systemic and pulmonary vascular resistance increased 43.9% (P < 0.001) and 204.2% (P < 0.001), respectively, after HBOC infusion. After blood exchange, the plasma hemoglobin level exhibited a circulatory half-life of 52.7 +/- 18.0 h. The formation of methemoglobin was significant, accounting for 33.0 +/- 7.1% of the total circulating plasma hemoglobin at 24 h; the half-life of HBOC capable of carrying oxygen was 30.1 +/- 5.4 h. This relatively short period of oxygen-carrying efficacy and the observed vasoconstriction properties may constrain the potential applications of HBOC solutions.