The effect of hypoxia on pulmonary blood flow (PBF) distribution were studied in 11 conscious resting rats. Microspheres were infused into the inferior vena cava during normoxia (Nx), acute normobaric hypoxia (AHx, 10% O2, 30 min), and 30 min after removal of hypoxia (post-hypoxia, PHx). The lungs were cut into 28 samples, and relative scatter of specific PBF was calculated as (sample activity/sample dry weight)/(total activity/total lung dry weight). Changes in pulmonary arterial pressure (PAP) during AHx were determined in five additional rats. During Nx, PBF was distributed preferentially to the hilar, central regions, rather than to the periphery. AHx resulted in a decrease in PaO2 from 85.1 +/- 0.9 to 37.8 +/- 1.2 Torr (mean +/- SE). Mean PAP increased significantly from 14.9 +/- 0.6 in Nx to 21.2 +/- 1.0 Torr in AHx (mean +/- SE). However, PBF distribution remained unchanged. PHx restored arterial blood gases and PAP to control levels without changing PBF distribution. The results indicate that conscious resting rats do not demonstrate changes in PBF distribution during AHx.