We assessed the red blood cell concentration (by estimating the cell number and hematocrit) in blood samples taken from the rabbit common carotid artery. This parameter was found to be dependent upon the blood flow velocity in the artery: reduction of the velocity to mean 62% of the initial value (100%), produced by partial obstruction of the artery, resulted in the decrease of red cell concentration by 1/4 to 1/7, despite the same systemic arterial pressure level and absence of appreciable changes in the blood taken from the control (contralateral) carotid artery. The relationship of the red cell number and hematocrit in the carotid artery was linear (r = 0.76). We conclude therefore, that changes in the local blood flow: hematocrit relationship induced by separation of red cells and plasma in vascular bifurcations occur not only in the microvascular beds, but in the entire arterial branching sequence, including the largest arteries.