The effect of haemodilution without volume expansion (isovolemic haemodilution) was assessed with respect to blood flow and oxygen transport across stenotic lesions of progressive severity in the dog carotid artery. As the mean haematocrit (Hct) was reduced from 40 +/- 1% (+/- SEM) to 32 +/- 0% (p less than 0.001), reductions in vascular resistance were significant across the 90% (p less than 0.001) and 95% (p less than 0.0003) relative carotid stenoses. Isovolemic haemodilution reduced fresh blood viscosity significantly by 27 +/- 3% (p less than 0.001) and 42 +/- 4% (p less than 0.001) at the low shear rates of 10 sec-1 and 1 sec-1 which are typical of low-flow states. Following a 20% reduction in Hct 30 to 35% increase (p less 0.001) in carotid blood flow occurred at non-critical degrees of stenosis while a mean 83% increase (p less than 0.001) occurred at the highly critical 95% relative stenosis. Oxygen transport after a 22% decrease in blood haemoglobin was significantly increased by 28% (p less than 0.01) at the 95% relative stenosis level. These data provide a physiologic rationale for the beneficial effects of haemodilution in acute cerebral ischaemia, cerebral vasospasm and cerebral revascularization or carotid endarterectomy.