In 13 healthy males pulmonary blood.flow distribution (PBFD), pulmonary artery pressure (PAP), and cardiac output (Q) were measured in the right lateral body position during 10 and 100% O2 breathing, and after maximal breath-holding preceded by breathing air or 100% O2. An increase in perfusion to the lower lung was n registered during oxygen breathing and after maximal breath-holding following oxygen breathing while no change occurred during hypoxia. Minor changes in PAP or Q seemed to influence the results very little.Changes in alveolar oxygen tension and opening of closed-off lung regions in the lower lung may explain the findings