Data obtained during positron emission tomographic (PET) imaging of the lung are expressed in terms of lung volume, not lung weight. Therefore changes in PET data may simply reflect changes in regional lung inflation. Normalizing PET data with regional measurements of lung density (rLD) made with positron transmission tomography may be useful, but how rLD is altered by changes in either regional inflation or blood volume has not been evaluated. In 12 supine dogs the mean rLD was 0.32±0.06 g/ml lung and was not significantly changed by doubling the tidal volume. The mean LD decreased with increasing levels of positive end-expiratory pressure (PEEP)(5–20 cm H 2 O), but regional changes were consistently present only at levels> 5 cm H 2 O. After sufficient hemorrhage to reduce cardiac output by half, mean rLD only decreased from 0.33±0.07 to 0.30±0.06 g/ml lung, while regional extravascular density remained unchanged. Thus, large changes in tidal volume, modest amounts of PEEP, and significant decreases in blood volume produced small, although measurable, decreases in rLD. Therefore, normalization of emission data with transmission derived measurements of LD should not be necessary for interventions that cause rLD to decrease.