We determined the spatial distribution of pulmonary blood flow at rest and during increasing levels of exercise (34, 59, and 90% of maximal oxygen consumption) in Thoroughbred racehorses (n = 4) using 15-mu m fluorescent microspheres. After the horses were killed, the lungs Mere flushed free of blood, removed, air-dried at total lung capacity, and sliced into isogravitational planes, which were sampled in a systematic fashion for three-dimensional reconstruction. The fluorescence was measured for quantification of blood flow Mean pulmonary blood flow heterogeneity (expressed as a coefficient of variation) did not change with increasing exercise levels [36.2 +/- 16.4 (rest) to 26.9 +/- 6.8% (gallop); P = not significant]. Greater than 70% of pulmonary blood flow variation across rest to high-exercise states is determined by a fixed spatial pattern. Thirty percent of the variation in pulmonary blood flow seen in horses over rest and exercising states is due to redistribution, The majority of flow redistribution was due to flow increasing to the dorsal region of the lung during exercise at 90% of maximal oxygen consumption (a flow gradient of 0.20 ml . min(-1). cm(-1) up the lung; P = 0.04).