We assessed the influence of cranial-to-caudal inertial force (+G(z)) and the countermeasures of anti-G suit and positive pressure breathing during G (PBG), specifically during +G(z), on regional pulmonary blood flow distribution. Unanesthetized swine were exposed randomly to 9 G, (resting), +3 G(z), +6 G(z), and +9 G(z), with and without anti-G suit and PEG with the use of the Air Force Research Laboratory centrifuge at Brooks Air Force Base (the gravitational force of the Earth, that is, the dorsal-to-ventral inertial force, was present for all runs). Fluorescent microspheres were injected into the pulmonary vasculature as a marker of regional pulmonary blood flow. Lungs were excised, dried, and diced into similar to 2-cm(3) pieces, and the fluorescence of each piece was measured. As +G(z) was increased from 0 to +3 G(z), blood flow shifted from cranial and hilar regions toward caudal and peripheral regions of the lung. This redistribution shifted back toward cranial and hilar regions as anti-G suit inflation pressure increased at +6 and +9 G(z). Perfusion heterogeneity increased with +G(z) stress and decreased at the higher anti-G suit pressures. The distribution of pulmonary blood flow was not affected by PEG. ANOVA indicated anatomic structure as the major determinant of pulmonary blood flow.