We studied 12 open-chest dogs to determine whether there are regional differences in left atrial distensibility. Sonomicrometer crystal pairs were used to measure the anteroposterior diameters of the left atrial body and appendage simultaneously over a wide range of intracardiac pressures and volumes obtained by intravenous saline infusion. Left atrial pressure-natural strain data for the body and appendage were fitted to an exponential function. The mean coefficient of the left atrial monoexponential pressure-strain relationship was greater for the body than appendage (3.1 +/- 1.1 vs. 1.9 +/- 0.7 mmHg, P less than 0.01). The Y-intercepts were not significantly different (5.2 +/- 1.7 vs 4.9 +/- 1.8 mmHg). The maximum minus minimum left atrial dimension, an index of the reservoir function of the atrium, increased with volume infusion in both the body and appendage and was significantly greater in the appendage than body at each level of left atrial pressure. Similarly, atrial systolic shortening fraction increased with volume infusion, and regional shortening was greater in the appendage than the body at each level of left atrial pressure. We conclude that regional differences in atrial distensibility exist in vivo and may play an important role in modulating systolic and diastolic function of the left atrium.