Serial studies of adaptation to aortic regurgitation (AR) were undertaken to determine whether sonomicrometry and echocardiography could be combined to measure changes in left ventricular (LV) cavitary volume (Vlv) and wall mass using the geometric formula [Vlv = K pi b2 alpha--wall volume], where K is a constant depending on the geometric model and a and be are epicardial major- and minor-axis diameters, respectively. Postmortem studies were performed in six normal dogs and in nine with AR; ultrasonic ventricular dimensions were measured as Vlv was varied with an intracavitary balloon. Three models were tested: 1) ellipsoid (model I; K = 1/6), 2) cylinder-ellipsoid (model II; K = 5/24), and 3) cylinder (model III; K = 1/4). The slope of the relationship between calculated Vlv and balloon volume varied between models (I, 0.71 +/- 0.11; II, 0.89 +/- 0.14; III, 1.07 +/- 0.17), and empiric determination of K to produce a slope of 1.0 resulted in a value of 0.26 +/- 0.04, not significantly different from the cylindrical model. Serial measurements of LV dimensions in 10 chronically instrumented conscious dogs revealed no significant change in end-diastolic or end-ejection LV shape after up to 16 wk of AR. Sonomicrometry and echocardiography can be integrated using a cylindrical geometric model to accurately estimate changes in end-diastolic or end-ejection Vlv during chronic volume overload.