The aim of this study was to find an ultrasonographic method to estimate pleural fluid volume in dogs. Nine canine cadavers of mixed breed were studied. Using a transsternal view, linear measurements from the pleural surface of the midline of the sternebra at the center of the heart to the furthest ventrolateral point of both right and left lung edges were recorded. Isotonic saline was injected using ultrasound guidance into both right and left pleural spaces and the measurements were repeated using standard increments until 1000 ml total volume was reached. No relationship was identified between mean distance and injected volume up to 100 ml. Thereafter, the mean distance increased in an approximately linear relationship with the cube root of fluid volume. There was a high correlation (r≥0.899) between the ultrasonographic measurement and fluid volume within individual dogs, but it was not possible to produce a useful equation to calculate absolute pleural fluid volume for new subjects. Nevertheless, ultrasonography may be used to semiquantitatively monitor pleural fluid volume, so that a decrease in the mean linear measurement obtained reflects a decrease in the total fluid volume.