We used computed tomography (CT) to examine the effects of infusing 60 ml/kg of saline into the pleural space of four anesthetized paralyzed dogs ventilated with a constant tidal volume at a positive end-expiratory pressure of 0.5 kPa. The dogs were positioned supine, and the thoracic cavity was scanned from apex to base before and immediately after effusate loading. Each CT image was analyzed semi-automatically on a 486 personal computer with custom-designed software. We found that, despite right-side infusion, the effusate was distributed bilaterally no doubt because of the incomplete canine mediastinum. In general, the volume change of the lung was one-third and that of the chest wall was two-thirds that of the total volume infused. Most of the lung volume was contained in the caudal one-third of the lung pre-effusion, and most of the lung volume loss due to effusion was from this same region. Chest wall volume increased and in a more uniform manner post-effusion. The decrease in lung volume resulted in an increase in the mean density of the lung and an increase in its vertical density gradient as the lung was lifted upward toward the sternum by the effusate. The lung lost vertical height while the chest wall increased both its vertical and lateral dimensions after effusate loading. These results suggest that expansion of the chest wall helps preserve lung volume in the presence of acute pleural effusion. We have also demonstrated that CT is a useful tool for assessing changes in volume, shape, and density of the respiratory system.