We examined the pulmonary blood flow distribution with intravenous fluorescent microspheres (15 [mu]m) in nine prone, unanesthetized, lambs. Lungs flushed free of blood, were air-dried at total lung capacity and sectioned into 2 cm3 pieces. The pieces were weighed, identified by lobe and assigned spatial coordinates. Fluorescence was read on a spectrophotometer, signals were corrected for piece weight and normalized to mean flow. Pulmonary blood flow heterogeneity was assessed using the coefficient of variation of the flow data. The number of pieces analyzed were 1249 +/- 150 (mean +/- SD) per animal. Heterogeneity of blood flow was 29.5 +/- 6.5% (coefficient of variation = SD/mean). Pulmonary blood flow decreased with distance from hilus (P<0.002), but did not change significantly with vertical height. Distance from the hilus was the best predictor of pulmonary blood flow (R2 = 0.201), and together with spatial coordinates and lobe accounted for 33.7 +/- 12.0% of blood flow variability. We conclude that pulmonary blood flow in the awake, prone sheep is distributed with a hilar-to-peripheral gradient, but no significant vertical gradient.