Single-photon emission-computed tomography (SPECT) on intact dogs and humans suggests that one aspect of regional blood flow in the lung (Qr) is independent of gravity, e.g., the gradient in Qr between the core and the periphery. To further evaluate these findings, six anesthetized healthy dogs (approximately 30 kg), two in the supine posture, two in the prone posture, and two suspended in the upright posture, breathing spontaneously, were injected (iv) at end expiration with 20 mCi99mTc-labeled albumin macroaggregates. The animals were killed, their chests were opened, their lungs were removed and dissected free of other tissue, and the blood was drained. The lungs were dried by blowing warm air (50 degrees C) while they were inflated to full capacity for about 18 h. The fully inflated and dry lungs were placed in the supine position and SPECT was performed to determine the three-dimensional distribution of activity. One hundred and twenty projections of the activity in the entire lungs were obtained at 3 degrees steps with a rotating gamma camera and stored in computer memory. Once SPECT was completed, either a coronal slice or a sagittal slice (1 cm thick) was cut and imaged directly by placing it against the gamma camera collimator for 6 min. The tomographic-reconstructed slices revealed that at isogravity, in all body postures, Qr in the central region of the lungs was up to 10 times that in the periphery. Furthermore, the central-peripheral gradient was discernible within the individual lobes. The direct images of slices also confirmed these findings. Although flow inequalities independent of gravity were present, the central region with the highest flow often was closer to the dependent regions of the lungs, suggesting that gravity had some influence on the final distribution. The results suggest that factors other than gravity also play an important role in the distribution of pulmonary blood flow. These factors may be related to the conductance of the vascular pathways that lead to different regions in the lungs.