Effects of vasoconstriction on the distribution of perfusion among alveoli are not well understood. To address this we used a new method we developed to determine how microvascular perfusion distribution was affected by a potent vasoconstrictor, the thromboxane receptor analog U46619. Our method was to infuse 4 µm diameter fluorescent latex microspheres into the circulation of isolated rat lungs vasoconstricted with U46619. We used a confocal microscope to image trapping patterns of the particles in dried sections of the lungs, then used Dispersion Index analysis to quantify the particle patterns in the images, which encompassed approximately 2,000 alveoli. Dispersion Indexes revealed significantly more particle clustering (inhomogeneous distribution) in vasoconstricted lungs than in normal flow controls or in controls in which flow was reduced by either lowering pulmonary artery pressure or raising left atrial pressure. These results suggest that vasoconstriction occurred in the microvessels themselves, which are much smaller vessels than those previously thought to be capable of vasoconstriction.