By diluting the hematocrit (Ha) in the rabbit's circulation without changing its blood volume, we found that the ventilatory-induced fluctuation (delta rho) in the density of aortic blood and Ha (which was in the range of 8-39%) are related by this linear regression: delta rho = 0.63 g/l (-0.009 + Ha). In this hemodilution experiment, the rabbits were ventilated by an intermittent positive pressure of 6 mmHg at a frequency of 30-35 cycles/min. Based on the Fahraeus effect for capillary blood flows and the dispersion of the density indicator in the rabbit's central circulation, we computed from the fluctuation of the measured density within a ventilation cycle the fluctuation of pulmonary capillary blood volume and found it to be 4.1 +/- 0.4% of the capillary blood volume for all hematocrits. Since the same fluctuation in the airway pressure was used to induce the volumetric fluctuation, its independence of Ha indicates that the hemodilution has no effect on the viscoelasticity of pulmonary capillaries.