Pulmonary blood flow (PBF) distribution was studied in 15 chronically hypoxic rats (3 weeks, 10% O-2 in N-2) breathing 10% O-2 (chronic hypoxia, CHx) and after 30 min of breathing air (acute normoxia, ANx). Controls were 15 normoxic littermates (normoxia, Nx) breathing air. Nonradioactive microspheres were infused into the inferior vena cava in the conscious resting state. 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). In Nx, PBF had a small but significant preferential distribution to the hilar, central regions, with lower blood flow to the peripheral regions (central-to-peripheral pattern). In CHx, however, there was no significant difference between blood hows to the central, middle and peripheral regions of the lung. ANx resulted in no change in PBF distribution. The results indicate that CHx attenuates the central-to-peripheral gradient of PBF distribution, probably due to vascular structural remodeling developed in CHx.