We employed a highly sensitive density measuring system to measure the decrease in blood density and plasma density due to the infusion of a saline bolus into the rabbit's circulation. Based on a vasculature model with the Fahraeus effect, we deduced a set of equations to calculate from the density decreases the red-blood-cell volume and plasma volume of the rabbit. A value of 0.82 +/- 0.02 was obtained for Fcell, the ratio of the hematocrit calculated from the two volumes and the arterial hematocrit.