Experimental brain injury attenuates the normal increase in cerebral blood flow (CBF) that accompanies a fall in Pao2, and this may contribute to the well-known detrimental effects of hypoxia following closed head injury. Anemia is also known to adversely affect posttraumatic survival, and it is reasonable to hypothesize that this too may be related to an altered cerebrovascular response. Therefore, to examine this possibility, pentobarbital-anesthetized rabbits were subjected to a left posterior parietal cryogenic cortical injury, followed 90 min later by isovolemic hemodilution with hetastarch. Unlesioned control animals underwent an identical degree of hemodilution. CBF was measured using radioactive microspheres. In control animals, hemodilution was accompanied by a marked increase in CBF in all brain regions. For example, in the left posterior cortex, CBF increased from 30 ± 14 mL/100 g/min (baseline Hct = 42 ± 2%, mean ± SD) to 100 ± 20 mL/100 g/min at Hct = 12 ± 1%. By contrast, there was a markedly attenuated response throughout the left (ipsilateral) hemisphere of injured animals, even in cortical regions distant from the injury. For example, in the left posterior cortex, CBF changed from a baseline of 32 ± 21 mL/100 g/min (baseline) to 40 ± 14 mL/100 g/min at the lowest Hct. CBF responses to hemodilution were unaltered in the contralateral hemisphere and in the hindbrain. These data indicate that a localized brain lesion can produce widespread ipsilateral alterations in the CBF response to hemodilution, with resultant compromise in cerebral 02 delivery. These data support the argument that the CBF increase produced by hemodilution is an active vasodilatory process rather than a passive response to changing blood viscosity.