The classical four-zone model of lung blood flow distribution has been questioned. We asked whether the effect of positive end-expiratory pressure (PEEP) is different between prone and supine position for lung tissue in the same zonal condition. Anesthetised and mechanically ventilated prone (n=6) and supine (n=5) sheep were studied at 0, 10 and 20 cmH₂O PEEP. Perfusion was measured with intravenous infusion of radiolabelled 15 µm microspheres. The right lung was dried at total lung capacity and diced into pieces (~1.5 cm³) keeping track of the spatial location of each piece. Radioactivity per unit weight was determined and normalized to the mean value for each condition and animal. In the supine posture perfusion to non-dependent lung regions decreased with little relative perfusion in non-dependent horizontal lung planes at 10 and 20 cm H₂O PEEP. In the prone position the effect of PEEP was markedly different with substantial perfusion remaining in non-dependent lung regions, and even increasing in these regions with 20 cm H₂O PEEP. Vertical blood flow gradients in zone II lung were large in supine, but surprisingly absent in prone animals. Isogravitational perfusion heterogeneity was smaller in prone than in supine animals at all PEEP levels. Redistribution of pulmonary perfusion by PEEP ventilation in supine was largely as predicted by the zonal model in marked contrast to the findings in prone. The differences between postures in blood flow distribution within zone II strongly indicate that factors in addition to pulmonary arterial, venous and alveolar pressure play important roles in determining perfusion distribution in the in situ lung. We suggest that regional variation in lung volume through the effect on vascular resistance is one such factor and that chest wall conformation and thoracic contents determines regional lung volume.