Solutions of modified cell-free hemoglobin, prepared from outdated red blood cells, have been developed during the past decade to circumvent the increasing need for allogeneic blood. Despite improvements in the safety and efficacy of these solutions, undesirable effects such as an increase in vascular tone leading to hypertension have not been fully resolved, which might hinder their clinical usefulness. To discriminate between the pharmacological and rheological effects of cell-free hemoglobin, we compared the effects of blood/cell-free hemoglobin mixtures of high versus low viscosity on hemodynamics and vascular hindrance, an index of vascular tone, which was normalized for blood viscosity. Anesthetized rats were subjected to 50% exchange transfusion with (1) high-viscosity solutions: whole blood (n=5) or red blood cells mixed with cell-free hemoglobin (Hb-Hv group, n=5); (2) low-viscosity solutions: cell-free hemoglobin (Hb-Lv group, n=5) or human albumin (n=5). Two hours after hemodilution, vascular hindrance remained unchanged in animals transfused with whole blood and albumin. Hb-Lv induced an immediate and sustained increase in vascular hindrance (208%). Conversely, in Hb-Hv animals, the vascular hindrance increase was delayed and smaller (27% to 147%), whereas peripheral resistance increased gradually (94% after 2 hours). Our results demonstrate the beneficial effects of cell-free hemoglobin in the presence of the animals' own red blood cells in maintaining physiological viscosity and limiting vasoconstriction because of the pharmacological properties of cell-free hemoglobin.