Pulmonary blood volume (PBV), flow, and pressures were measured in 15 cardiopulmonary normal subjects, while supine and at 60 degrees head-up passive tilt. PBV, cardiac index (CI), right ventricular diastolic volume (VDVR), and mean pulmonary arterial (Ppa), venous (Pv), and mean right atrial (Pra) pressures all decreased during the first 4 min of tilting, with little subsequent change thereafter, to 30 min. Relative changes of CI, PBV, and VDVR were of similar magnitude (25--30%; mean Ppa and Pv fell 3.2 and 2.3 Torr, respectively. From measurements of thoracic dimensions and changes in intravascular pressures, we calculate that 11% of the pulmonary vascular bed falls into zone I condition, 34% goes from zone III to zone II, and 55% remains in zone III condition during tilt. A highly significant correlation exists between PBV and CI (P less than 0.001), with no significant change in pulmonary mean transit time during tilt. Significant correlations also exist between PBV and Pra (P less than 0.001), Pv (P less than 0.01), and Ppa (P less than 0.05). We conclude that outflow pressure plays a significant role in determining PBV during head-up tilt, but the major determinant is venous return to the right heart.