Left ventricular (LV) volume can be estimated from three orthogonal dimensions measured by sonomicrometry. Often, an index based on fewer than three dimensions has been substituted for volume. We consider whether a consistent relationship between LV cross-sectional area, computed as the product of the minor axes dimensions and LV three-dimensional volume, is maintained throughout the responses to application and release of vena caval, pulmonary artery, and aortic occlusions, which were held for approximately 30 beats. In six dogs, the relationship between area and volume was highly linear, with an average correlation of 0.98 and standard error of the estimate of 0.9 ml. Within each dog, there were small but statistically significant differences in the intercepts in the regression lines among the three interventions. However, the magnitude of these differences averaged only 0.5 ml. There was not a systematic difference between the relationship for vena caval and pulmonary artery occlusions, and the relationship for aortic occlusions shifted upward by an average of only 0.8 ml. We conclude that cross-sectional area can be substituted for the three-dimensional volume during the transient responses to acute alterations in the external load conditions.