The detection of small amounts of free peritoneal gas in the canine patient can pose a diagnostic dilemma. The objective of this study was to determine how much of this free gas could be detected ultrasonographically. Ultrasound examinations were carried out after increments (0.1 ml) of air were intraperitoneally injected. Via ultrasonography, 0.4 ml of free gas can be reliably detected. The authors concluded that ultrasonography is a very sensitive modality for the detection of small amounts of peritoneal free gas (above 0.4 ml).