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Recent reports indicate that in cases of elevated plasma or blood viscosity, red cell velocity through the cerebral microcirculation is unimpaired while plasma transit is retarded. One explanation may be that plasma skimming is increased, with increased shunting of plasma into longer, less direct flow paths, while an increasing proportion of red cells take shorter paths. Such inhomogeneities of flow on a microvascular level can be directly demonstrated in cases of endothelial damage or anoxia, and may produce an altered regional hematocrit.Measurements of regional cerebral blood flow are dependent upon regional hematocrit because this affects the tissue:blood partition coefficient (λ) of diffusible indicators. Hence changes in regional HCT may produce errors in the calculation of regional blood flow unless the value of λ. is corrected to reflect altered HCT. Moreover, no matter what kind of indicator is used, our functional interpretation of regional blood flow measurements is dependent upon our assumption that the ‘blood’ in the region maintains a constant capacity to carry nutrients like oxygen. Thus the lack of a constant relationship between plasma flow and red cell flow may produce errors either in the measurement or interpretation of regional flow measurements. These errors will increase in importance as blood flow is measured in smaller and smaller volumes of brain.