The distribution of blood flow in the pulmonary vascular bed under +GX (forward or transverse acceleration) was studied by intravenous injection of radioactive iodine 131-macroaggregated albumin (I131-MAA) in 3 normal subjects while they were under +1 Gx, +4 Gx, and +8 Gx on a human centrifuge. The resting distribution of radioactivity in the lungs, representing the distribution of pulmonary blood flow at the time of injection, was assessed 1-3 hr. later by lateral radioisotope scanning. The distribution of pulmonary blood flow was not markedly different at +1 Gx, +4 Gx, and +8 Gx despite a difference between anterior and posterior pulmonary arterial pressures estimated to be 88 mm Hg under +8 Gx. These findings indicate that under +GX (forward or transverse acceleration), unlike +GZ (headward or positive acceleration), the distribution of pulmonary blood flow is not markedly altered and the regional flow of blood in the lung may not be significantly changed by high intravascular pressures.